From 1b9f37302761ec02375e52f0f5563cc868295b25 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Tue, 27 Jan 2026 23:39:52 +0000
Subject: [PATCH 01/29] gh-144278: Enable defining _PY_DISABLE_SYS_CACHE_TAG to
 disable sys.implementation.cache_tag. This has the effect of disabling
 automatic .pyc caching.

---
 .../next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst | 4 ++++
 1 file changed, 4 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst

diff --git a/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst b/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst
new file mode 100644
index 00000000000000..fa6b0c6e505cdb
--- /dev/null
+++ b/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst
@@ -0,0 +1,4 @@
+Enables patching in a ``_PY_DISABLE_SYS_CACHE_TAG`` preprocessor definition
+when building to force :data:`sys.implementation.cache_tag` to ``None``.
+This has the effect of completely disabling automatic creation and reading
+of ``.pyc`` files.

From 732c090b3f280bdf9b2c53185785c5dcd22049d4 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Tue, 27 Jan 2026 23:41:10 +0000
Subject: [PATCH 02/29] Helps if you commit the actual changes...

---
 Lib/py_compile.py                             |   4 +-
 Lib/test/support/import_helper.py             |  21 ++-
 Lib/test/test_argparse.py                     |   3 +-
 Lib/test/test_capi/test_import.py             |   5 +-
 Lib/test/test_cmd_line_script.py              |  21 ++-
 Lib/test/test_compileall.py                   |   4 +
 Lib/test/test_import/__init__.py              |  28 ++--
 .../test_importlib/source/test_file_loader.py |  19 ++-
 Lib/test/test_importlib/source/test_finder.py |   2 +
 Lib/test/test_importlib/test_abc.py           |  12 +-
 Lib/test/test_importlib/test_api.py           |   4 -
 Lib/test/test_importlib/test_pkg_import.py    |   9 +-
 Lib/test/test_importlib/test_spec.py          |  17 ++-
 Lib/test/test_importlib/test_util.py          |  30 +++++
 Lib/test/test_importlib/util.py               |   3 +
 Lib/test/test_inspect/test_inspect.py         |   3 +-
 .../test_multiprocessing_main_handling.py     |  12 +-
 Lib/test/test_py_compile.py                   |  68 +++++++---
 Lib/test/test_pydoc/test_pydoc.py             |   6 +-
 Lib/test/test_reprlib.py                      |   7 +-
 Lib/test/test_runpy.py                        |  27 ++--
 Lib/test/test_zipimport.py                    |   1 -
 Lib/zipfile/__init__.py                       | 122 ++++++------------
 Modules/_testlimitedcapi/import.c             |   2 +-
 PC/pyconfig.h                                 |   3 +
 Python/sysmodule.c                            |  11 +-
 26 files changed, 270 insertions(+), 174 deletions(-)

diff --git a/Lib/py_compile.py b/Lib/py_compile.py
index 43d8ec90ffb6b1..694ea9304da9f9 100644
--- a/Lib/py_compile.py
+++ b/Lib/py_compile.py
@@ -194,8 +194,10 @@ def main():
     else:
         filenames = args.filenames
     for filename in filenames:
+        cfilename = (None if sys.implementation.cache_tag
+                     else f"{filename.rpartition('.')[0]}.pyc")
         try:
-            compile(filename, doraise=True)
+            compile(filename, cfilename, doraise=True)
         except PyCompileError as error:
             if args.quiet:
                 parser.exit(1)
diff --git a/Lib/test/support/import_helper.py b/Lib/test/support/import_helper.py
index 4c7eac0b7eb674..f1642e51daf815 100644
--- a/Lib/test/support/import_helper.py
+++ b/Lib/test/support/import_helper.py
@@ -4,6 +4,7 @@
 import importlib.machinery
 import importlib.util
 import os
+import py_compile
 import shutil
 import sys
 import textwrap
@@ -49,20 +50,30 @@ def forget(modname):
         # combinations of PEP 3147/488 and legacy pyc files.
         unlink(source + 'c')
         for opt in ('', 1, 2):
-            unlink(importlib.util.cache_from_source(source, optimization=opt))
+            try:
+                unlink(importlib.util.cache_from_source(source, optimization=opt))
+            except NotImplementedError:
+                pass
 
 
-def make_legacy_pyc(source):
+def make_legacy_pyc(source, allow_compile=False):
     """Move a PEP 3147/488 pyc file to its legacy pyc location.
 
     :param source: The file system path to the source file.  The source file
-        does not need to exist, however the PEP 3147/488 pyc file must exist.
+        does not need to exist, however the PEP 3147/488 pyc file must exist or
+        allow_compile must be set.
+    :param allow_compile: If True, uses py_compile to create a .pyc if it does
+        not exist. This should be passed as True if cache_tag may be None.
     :return: The file system path to the legacy pyc file.
     """
-    pyc_file = importlib.util.cache_from_source(source)
     assert source.endswith('.py')
     legacy_pyc = source + 'c'
-    shutil.move(pyc_file, legacy_pyc)
+    try:
+        pyc_file = importlib.util.cache_from_source(source)
+    except NotImplementedError:
+        py_compile.compile(source, legacy_pyc, doraise=True)
+    else:
+        shutil.move(pyc_file, legacy_pyc)
     return legacy_pyc
 
 
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index 77170244675474..d2e9903f897bed 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -7162,9 +7162,8 @@ def make_script(self, dirname, basename, *, compiled=False):
         script_name = script_helper.make_script(dirname, basename, self.source)
         if not compiled:
             return script_name
-        py_compile.compile(script_name, doraise=True)
+        pyc_file = import_helper.make_legacy_pyc(script_name, allow_compile=True)
         os.remove(script_name)
-        pyc_file = import_helper.make_legacy_pyc(script_name)
         return pyc_file
 
     def make_zip_script(self, script_name, name_in_zip=None):
diff --git a/Lib/test/test_capi/test_import.py b/Lib/test/test_capi/test_import.py
index 57e0316fda8a52..ea845df75ef3a7 100644
--- a/Lib/test/test_capi/test_import.py
+++ b/Lib/test/test_capi/test_import.py
@@ -289,7 +289,10 @@ def check_executecode_pathnames(self, execute_code_func, object=False):
             self.check_executecodemodule(execute_code_func, NULL, pathname)
 
         # Test NULL pathname and non-NULL cpathname
-        pyc_filename = importlib.util.cache_from_source(__file__)
+        try:
+            pyc_filename = importlib.util.cache_from_source(__file__)
+        except NotImplementedError:
+            return
         py_filename = importlib.util.source_from_cache(pyc_filename)
         origin = self.check_executecodemodule(execute_code_func, NULL, pyc_filename)
         if not object:
diff --git a/Lib/test/test_cmd_line_script.py b/Lib/test/test_cmd_line_script.py
index 8695df9eb0c294..73b1f671c58555 100644
--- a/Lib/test/test_cmd_line_script.py
+++ b/Lib/test/test_cmd_line_script.py
@@ -240,9 +240,8 @@ def test_script_abspath(self):
     def test_script_compiled(self):
         with os_helper.temp_dir() as script_dir:
             script_name = _make_test_script(script_dir, 'script')
-            py_compile.compile(script_name, doraise=True)
+            pyc_file = import_helper.make_legacy_pyc(script_name, allow_compile=True)
             os.remove(script_name)
-            pyc_file = import_helper.make_legacy_pyc(script_name)
             self._check_script(pyc_file, pyc_file,
                                pyc_file, script_dir, None,
                                importlib.machinery.SourcelessFileLoader)
@@ -257,9 +256,8 @@ def test_directory(self):
     def test_directory_compiled(self):
         with os_helper.temp_dir() as script_dir:
             script_name = _make_test_script(script_dir, '__main__')
-            py_compile.compile(script_name, doraise=True)
+            pyc_file = import_helper.make_legacy_pyc(script_name, allow_compile=True)
             os.remove(script_name)
-            pyc_file = import_helper.make_legacy_pyc(script_name)
             self._check_script(script_dir, pyc_file, script_dir,
                                script_dir, '',
                                importlib.machinery.SourcelessFileLoader)
@@ -279,8 +277,8 @@ def test_zipfile(self):
     def test_zipfile_compiled_timestamp(self):
         with os_helper.temp_dir() as script_dir:
             script_name = _make_test_script(script_dir, '__main__')
-            compiled_name = py_compile.compile(
-                script_name, doraise=True,
+            compiled_name = script_name + 'c'
+            py_compile.compile(script_name, compiled_name, doraise=True,
                 invalidation_mode=py_compile.PycInvalidationMode.TIMESTAMP)
             zip_name, run_name = make_zip_script(script_dir, 'test_zip', compiled_name)
             self._check_script(zip_name, run_name, zip_name, zip_name, '',
@@ -289,8 +287,8 @@ def test_zipfile_compiled_timestamp(self):
     def test_zipfile_compiled_checked_hash(self):
         with os_helper.temp_dir() as script_dir:
             script_name = _make_test_script(script_dir, '__main__')
-            compiled_name = py_compile.compile(
-                script_name, doraise=True,
+            compiled_name = script_name + 'c'
+            py_compile.compile(script_name, compiled_name, doraise=True,
                 invalidation_mode=py_compile.PycInvalidationMode.CHECKED_HASH)
             zip_name, run_name = make_zip_script(script_dir, 'test_zip', compiled_name)
             self._check_script(zip_name, run_name, zip_name, zip_name, '',
@@ -299,8 +297,8 @@ def test_zipfile_compiled_checked_hash(self):
     def test_zipfile_compiled_unchecked_hash(self):
         with os_helper.temp_dir() as script_dir:
             script_name = _make_test_script(script_dir, '__main__')
-            compiled_name = py_compile.compile(
-                script_name, doraise=True,
+            compiled_name = script_name + 'c'
+            py_compile.compile(script_name, compiled_name, doraise=True,
                 invalidation_mode=py_compile.PycInvalidationMode.UNCHECKED_HASH)
             zip_name, run_name = make_zip_script(script_dir, 'test_zip', compiled_name)
             self._check_script(zip_name, run_name, zip_name, zip_name, '',
@@ -353,9 +351,8 @@ def test_package_compiled(self):
             pkg_dir = os.path.join(script_dir, 'test_pkg')
             make_pkg(pkg_dir)
             script_name = _make_test_script(pkg_dir, '__main__')
-            compiled_name = py_compile.compile(script_name, doraise=True)
+            pyc_file = import_helper.make_legacy_pyc(script_name, allow_compile=True)
             os.remove(script_name)
-            pyc_file = import_helper.make_legacy_pyc(script_name)
             self._check_script(["-m", "test_pkg"], pyc_file,
                                pyc_file, script_dir, 'test_pkg',
                                importlib.machinery.SourcelessFileLoader,
diff --git a/Lib/test/test_compileall.py b/Lib/test/test_compileall.py
index 8384c183dd92dd..b2150b621516ba 100644
--- a/Lib/test/test_compileall.py
+++ b/Lib/test/test_compileall.py
@@ -33,6 +33,10 @@
 from test.support.os_helper import FakePath
 
 
+if sys.implementation.cache_tag is None:
+    raise unittest.SkipTest('requires sys.implementation.cache_tag is not None')
+
+
 def get_pyc(script, opt):
     if not opt:
         # Replace None and 0 with ''
diff --git a/Lib/test/test_import/__init__.py b/Lib/test/test_import/__init__.py
index 59c6dc4587c93d..e26c0f87a4de7f 100644
--- a/Lib/test/test_import/__init__.py
+++ b/Lib/test/test_import/__init__.py
@@ -76,7 +76,7 @@
 
 
 skip_if_dont_write_bytecode = unittest.skipIf(
-        sys.dont_write_bytecode,
+        sys.dont_write_bytecode or sys.implementation.cache_tag is None,
         "test meaningful only when writing bytecode")
 
 
@@ -504,7 +504,7 @@ def test_module_with_large_stack(self, module='longlist'):
         try:
             # Compile & remove .py file; we only need .pyc.
             # Bytecode must be relocated from the PEP 3147 bytecode-only location.
-            py_compile.compile(filename)
+            make_legacy_pyc(filename, allow_compile=True)
         finally:
             unlink(filename)
 
@@ -514,7 +514,6 @@ def test_module_with_large_stack(self, module='longlist'):
 
         namespace = {}
         try:
-            make_legacy_pyc(filename)
             # This used to crash.
             exec('import ' + module, None, namespace)
         finally:
@@ -1399,7 +1398,10 @@ def func():
 """
     dir_name = os.path.abspath(TESTFN)
     file_name = os.path.join(dir_name, module_name) + os.extsep + "py"
-    compiled_name = importlib.util.cache_from_source(file_name)
+    try:
+        compiled_name = importlib.util.cache_from_source(file_name)
+    except NotImplementedError:
+        compiled_name = None
 
     def setUp(self):
         self.sys_path = sys.path[:]
@@ -1417,7 +1419,8 @@ def tearDown(self):
         else:
             unload(self.module_name)
         unlink(self.file_name)
-        unlink(self.compiled_name)
+        if self.compiled_name:
+            unlink(self.compiled_name)
         rmtree(self.dir_name)
 
     def import_module(self):
@@ -1436,6 +1439,8 @@ def test_basics(self):
         self.assertEqual(mod.code_filename, self.file_name)
         self.assertEqual(mod.func_filename, self.file_name)
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag is not None')
     def test_incorrect_code_name(self):
         py_compile.compile(self.file_name, dfile="another_module.py")
         mod = self.import_module()
@@ -1445,9 +1450,9 @@ def test_incorrect_code_name(self):
 
     def test_module_without_source(self):
         target = "another_module.py"
-        py_compile.compile(self.file_name, dfile=target)
+        pyc_file = self.file_name + 'c'
+        py_compile.compile(self.file_name, pyc_file, dfile=target)
         os.remove(self.file_name)
-        pyc_file = make_legacy_pyc(self.file_name)
         importlib.invalidate_caches()
         mod = self.import_module()
         self.assertEqual(mod.module_filename, pyc_file)
@@ -1455,8 +1460,9 @@ def test_module_without_source(self):
         self.assertEqual(mod.func_filename, target)
 
     def test_foreign_code(self):
-        py_compile.compile(self.file_name)
-        with open(self.compiled_name, "rb") as f:
+        compiled_name = self.compiled_name or (self.file_name + 'c')
+        py_compile.compile(self.file_name, compiled_name)
+        with open(compiled_name, "rb") as f:
             header = f.read(16)
             code = marshal.load(f)
         constants = list(code.co_consts)
@@ -1464,9 +1470,11 @@ def test_foreign_code(self):
         pos = constants.index(1000)
         constants[pos] = foreign_code
         code = code.replace(co_consts=tuple(constants))
-        with open(self.compiled_name, "wb") as f:
+        with open(compiled_name, "wb") as f:
             f.write(header)
             marshal.dump(code, f)
+        if not self.compiled_name:
+            os.remove(self.file_name)
         mod = self.import_module()
         self.assertEqual(mod.constant.co_filename, foreign_code.co_filename)
 
diff --git a/Lib/test/test_importlib/source/test_file_loader.py b/Lib/test/test_importlib/source/test_file_loader.py
index 5d5d4722171a8e..e4bd850f3514ff 100644
--- a/Lib/test/test_importlib/source/test_file_loader.py
+++ b/Lib/test/test_importlib/source/test_file_loader.py
@@ -213,12 +213,21 @@ def manipulate_bytecode(self,
             del sys.modules['_temp']
         except KeyError:
             pass
-        py_compile.compile(mapping[name], invalidation_mode=invalidation_mode)
-        if not del_source:
-            bytecode_path = self.util.cache_from_source(mapping[name])
+        if sys.implementation.cache_tag is None:
+            if del_source:
+                bytecode_path = mapping[name] + 'c'
+                py_compile.compile(mapping[name], bytecode_path,
+                                   invalidation_mode=invalidation_mode)
+                os.unlink(mapping[name])
+            else:
+                raise unittest.SkipTest('requires sys.implementation.cache_tag')
         else:
-            os.unlink(mapping[name])
-            bytecode_path = make_legacy_pyc(mapping[name])
+            py_compile.compile(mapping[name], invalidation_mode=invalidation_mode)
+            if not del_source:
+                bytecode_path = self.util.cache_from_source(mapping[name])
+            else:
+                os.unlink(mapping[name])
+                bytecode_path = make_legacy_pyc(mapping[name])
         if manipulator:
             with open(bytecode_path, 'rb') as file:
                 bc = file.read()
diff --git a/Lib/test/test_importlib/source/test_finder.py b/Lib/test/test_importlib/source/test_finder.py
index c33e90232b36e6..91865b997ad364 100644
--- a/Lib/test/test_importlib/source/test_finder.py
+++ b/Lib/test/test_importlib/source/test_finder.py
@@ -57,6 +57,8 @@ def run_test(self, test, create=None, *, compile_=None, unlink=None):
         """
         if create is None:
             create = {test}
+        if (compile_ or unlink) and sys.implementation.cache_tag is None:
+            raise unittest.SkipTest('requires sys.implementation.cache_tag')
         with util.create_modules(*create) as mapping:
             if compile_:
                 for name in compile_:
diff --git a/Lib/test/test_importlib/test_abc.py b/Lib/test/test_importlib/test_abc.py
index 7c146ea853b0d9..b24c576646842f 100644
--- a/Lib/test/test_importlib/test_abc.py
+++ b/Lib/test/test_importlib/test_abc.py
@@ -533,7 +533,10 @@ class SourceLoader(SourceOnlyLoader):
 
     def __init__(self, path, magic=None):
         super().__init__(path)
-        self.bytecode_path = self.util.cache_from_source(self.path)
+        try:
+            self.bytecode_path = self.util.cache_from_source(self.path)
+        except NotImplementedError:
+            self.bytecode_path = None
         self.source_size = len(self.source)
         if magic is None:
             magic = self.util.MAGIC_NUMBER
@@ -579,7 +582,10 @@ def setUp(self, *, is_package=True, **kwargs):
             module_name = 'mod'
             self.path = os.path.join(self.package, '.'.join(['mod', 'py']))
             self.name = '.'.join([self.package, module_name])
-        self.cached = self.util.cache_from_source(self.path)
+        try:
+            self.cached = self.util.cache_from_source(self.path)
+        except NotImplementedError:
+            self.cached = None
         self.loader = self.loader_mock(self.path, **kwargs)
 
     def verify_module(self, module):
@@ -656,6 +662,8 @@ def test_get_source_encoding(self):
 
 
 @unittest.skipIf(sys.dont_write_bytecode, "sys.dont_write_bytecode is true")
+@unittest.skipIf(sys.implementation.cache_tag is None,
+                 "sys.implementation.cache_tag is None")
 class SourceLoaderBytecodeTests(SourceLoaderTestHarness):
 
     """Test importlib.abc.SourceLoader's use of bytecode.
diff --git a/Lib/test/test_importlib/test_api.py b/Lib/test/test_importlib/test_api.py
index 4de0cf029a81e0..70d93d693ae593 100644
--- a/Lib/test/test_importlib/test_api.py
+++ b/Lib/test/test_importlib/test_api.py
@@ -231,7 +231,6 @@ def test_reload_location_changed(self):
                     # Start as a plain module.
                     self.init.invalidate_caches()
                     path = os.path.join(cwd, name + '.py')
-                    cached = self.util.cache_from_source(path)
                     expected = {'__name__': name,
                                 '__package__': '',
                                 '__file__': path,
@@ -251,7 +250,6 @@ def test_reload_location_changed(self):
                     # Change to a package.
                     self.init.invalidate_caches()
                     init_path = os.path.join(cwd, name, '__init__.py')
-                    cached = self.util.cache_from_source(init_path)
                     expected = {'__name__': name,
                                 '__package__': name,
                                 '__file__': init_path,
@@ -281,7 +279,6 @@ def test_reload_namespace_changed(self):
                     # Start as a namespace package.
                     self.init.invalidate_caches()
                     bad_path = os.path.join(cwd, name, '__init.py')
-                    cached = self.util.cache_from_source(bad_path)
                     expected = {'__name__': name,
                                 '__package__': name,
                                 '__doc__': None,
@@ -310,7 +307,6 @@ def test_reload_namespace_changed(self):
                     # Change to a regular package.
                     self.init.invalidate_caches()
                     init_path = os.path.join(cwd, name, '__init__.py')
-                    cached = self.util.cache_from_source(init_path)
                     expected = {'__name__': name,
                                 '__package__': name,
                                 '__file__': init_path,
diff --git a/Lib/test/test_importlib/test_pkg_import.py b/Lib/test/test_importlib/test_pkg_import.py
index 5ffae6222bacb8..287684efc85a91 100644
--- a/Lib/test/test_importlib/test_pkg_import.py
+++ b/Lib/test/test_importlib/test_pkg_import.py
@@ -39,9 +39,12 @@ def tearDown(self):
         self.remove_modules()
 
     def rewrite_file(self, contents):
-        compiled_path = cache_from_source(self.module_path)
-        if os.path.exists(compiled_path):
-            os.remove(compiled_path)
+        try:
+            compiled_path = cache_from_source(self.module_path)
+            if os.path.exists(compiled_path):
+                os.remove(compiled_path)
+        except NotImplementedError:
+            pass
         with open(self.module_path, 'w', encoding='utf-8') as f:
             f.write(contents)
 
diff --git a/Lib/test/test_importlib/test_spec.py b/Lib/test/test_importlib/test_spec.py
index b48d0a101ca9e7..77b6228940c3b9 100644
--- a/Lib/test/test_importlib/test_spec.py
+++ b/Lib/test/test_importlib/test_spec.py
@@ -52,7 +52,10 @@ class ModuleSpecTests:
     def setUp(self):
         self.name = 'spam'
         self.path = 'spam.py'
-        self.cached = self.util.cache_from_source(self.path)
+        try:
+            self.cached = self.util.cache_from_source(self.path)
+        except NotImplementedError:
+            self.cached = None
         self.loader = TestLoader()
         self.spec = self.machinery.ModuleSpec(self.name, self.loader)
         self.loc_spec = self.machinery.ModuleSpec(self.name, self.loader,
@@ -184,6 +187,8 @@ def test_cached_with_origin_not_location(self):
 
         self.assertIs(spec.cached, None)
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     "sys.implementation.cache_tag is None")
     def test_cached_source(self):
         expected = self.util.cache_from_source(self.path)
 
@@ -224,7 +229,10 @@ def bootstrap(self):
     def setUp(self):
         self.name = 'spam'
         self.path = 'spam.py'
-        self.cached = self.util.cache_from_source(self.path)
+        try:
+            self.cached = self.util.cache_from_source(self.path)
+        except NotImplementedError:
+            self.cached = None
         self.loader = TestLoader()
         self.spec = self.machinery.ModuleSpec(self.name, self.loader)
         self.loc_spec = self.machinery.ModuleSpec(self.name, self.loader,
@@ -349,7 +357,10 @@ class FactoryTests:
     def setUp(self):
         self.name = 'spam'
         self.path = os.path.abspath('spam.py')
-        self.cached = self.util.cache_from_source(self.path)
+        try:
+            self.cached = self.util.cache_from_source(self.path)
+        except NotImplementedError:
+            self.cached = None
         self.loader = TestLoader()
         self.fileloader = TestLoader(self.path)
         self.pkgloader = TestLoader(self.path, True)
diff --git a/Lib/test/test_importlib/test_util.py b/Lib/test/test_importlib/test_util.py
index 17a211f10fa0ac..a926a7a4d408af 100644
--- a/Lib/test/test_importlib/test_util.py
+++ b/Lib/test/test_importlib/test_util.py
@@ -345,6 +345,8 @@ def test_cache_from_source_no_cache_tag(self):
             with self.assertRaises(NotImplementedError):
                 self.util.cache_from_source('whatever.py')
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_cache_from_source_no_dot(self):
         # Directory with a dot, filename without dot.
         path = os.path.join('foo.bar', 'file')
@@ -353,12 +355,16 @@ def test_cache_from_source_no_dot(self):
         self.assertEqual(self.util.cache_from_source(path, optimization=''),
                          expect)
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_cache_from_source_cwd(self):
         path = 'foo.py'
         expect = os.path.join('__pycache__', 'foo.{}.pyc'.format(self.tag))
         self.assertEqual(self.util.cache_from_source(path, optimization=''),
                          expect)
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_cache_from_source_optimization_empty_string(self):
         # Setting 'optimization' to '' leads to no optimization tag (PEP 488).
         path = 'foo.py'
@@ -366,6 +372,8 @@ def test_cache_from_source_optimization_empty_string(self):
         self.assertEqual(self.util.cache_from_source(path, optimization=''),
                          expect)
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_cache_from_source_optimization_None(self):
         # Setting 'optimization' to None uses the interpreter's optimization.
         # (PEP 488)
@@ -382,6 +390,8 @@ def test_cache_from_source_optimization_None(self):
         self.assertEqual(self.util.cache_from_source(path, optimization=None),
                          expect)
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_cache_from_source_optimization_set(self):
         # The 'optimization' parameter accepts anything that has a string repr
         # that passes str.alnum().
@@ -399,6 +409,8 @@ def test_cache_from_source_optimization_set(self):
         with self.assertRaises(ValueError):
             self.util.cache_from_source(path, optimization='path/is/bad')
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_cache_from_source_debug_override_optimization_both_set(self):
         # Can only set one of the optimization-related parameters.
         with warnings.catch_warnings():
@@ -408,6 +420,8 @@ def test_cache_from_source_debug_override_optimization_both_set(self):
 
     @unittest.skipUnless(os.sep == '\\' and os.altsep == '/',
                      'test meaningful only where os.altsep is defined')
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_sep_altsep_and_sep_cache_from_source(self):
         # Windows path and PEP 3147 where sep is right of altsep.
         self.assertEqual(
@@ -440,44 +454,60 @@ def test_source_from_cache_no_cache_tag(self):
             with self.assertRaises(NotImplementedError):
                 self.util.source_from_cache(path)
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_source_from_cache_bad_path(self):
         # When the path to a pyc file is not in PEP 3147 format, a ValueError
         # is raised.
         self.assertRaises(
             ValueError, self.util.source_from_cache, '/foo/bar/bazqux.pyc')
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_source_from_cache_no_slash(self):
         # No slashes at all in path -> ValueError
         self.assertRaises(
             ValueError, self.util.source_from_cache, 'foo.cpython-32.pyc')
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_source_from_cache_too_few_dots(self):
         # Too few dots in final path component -> ValueError
         self.assertRaises(
             ValueError, self.util.source_from_cache, '__pycache__/foo.pyc')
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_source_from_cache_too_many_dots(self):
         with self.assertRaises(ValueError):
             self.util.source_from_cache(
                     '__pycache__/foo.cpython-32.opt-1.foo.pyc')
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_source_from_cache_not_opt(self):
         # Non-`opt-` path component -> ValueError
         self.assertRaises(
             ValueError, self.util.source_from_cache,
             '__pycache__/foo.cpython-32.foo.pyc')
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_source_from_cache_no__pycache__(self):
         # Another problem with the path -> ValueError
         self.assertRaises(
             ValueError, self.util.source_from_cache,
             '/foo/bar/foo.cpython-32.foo.pyc')
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_source_from_cache_optimized_bytecode(self):
         # Optimized bytecode is not an issue.
         path = os.path.join('__pycache__', 'foo.{}.opt-1.pyc'.format(self.tag))
         self.assertEqual(self.util.source_from_cache(path), 'foo.py')
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
     def test_source_from_cache_missing_optimization(self):
         # An empty optimization level is a no-no.
         path = os.path.join('__pycache__', 'foo.{}.opt-.pyc'.format(self.tag))
diff --git a/Lib/test/test_importlib/util.py b/Lib/test/test_importlib/util.py
index efbec667317d5f..6399f952f9e912 100644
--- a/Lib/test/test_importlib/util.py
+++ b/Lib/test/test_importlib/util.py
@@ -292,6 +292,9 @@ def writes_bytecode_files(fxn):
     tests that require it to be set to False."""
     if sys.dont_write_bytecode:
         return unittest.skip("relies on writing bytecode")(fxn)
+    if sys.implementation.cache_tag is None:
+        return unittest.skip("requires sys.implementation.cache_tag to not be None")(fxn)
+
     @functools.wraps(fxn)
     def wrapper(*args, **kwargs):
         original = sys.dont_write_bytecode
diff --git a/Lib/test/test_inspect/test_inspect.py b/Lib/test/test_inspect/test_inspect.py
index 1999aa770ecc56..e4a3a7d9add2c2 100644
--- a/Lib/test/test_inspect/test_inspect.py
+++ b/Lib/test/test_inspect/test_inspect.py
@@ -6527,7 +6527,8 @@ def test_details(self):
         self.assertIn(module.__name__, output)
         self.assertIn(module.__spec__.origin, output)
         self.assertIn(module.__file__, output)
-        self.assertIn(module.__spec__.cached, output)
+        if module.__spec__.cached:
+            self.assertIn(module.__spec__.cached, output)
         self.assertEqual(err, b'')
 
 
diff --git a/Lib/test/test_multiprocessing_main_handling.py b/Lib/test/test_multiprocessing_main_handling.py
index 6b30a89316703b..fbd44fb0d3d5eb 100644
--- a/Lib/test/test_multiprocessing_main_handling.py
+++ b/Lib/test/test_multiprocessing_main_handling.py
@@ -196,9 +196,8 @@ def test_ipython_workaround(self):
     def test_script_compiled(self):
         with os_helper.temp_dir() as script_dir:
             script_name = _make_test_script(script_dir, 'script')
-            py_compile.compile(script_name, doraise=True)
+            pyc_file = import_helper.make_legacy_pyc(script_name, allow_compile=True)
             os.remove(script_name)
-            pyc_file = import_helper.make_legacy_pyc(script_name)
             self._check_script(pyc_file)
 
     def test_directory(self):
@@ -213,9 +212,8 @@ def test_directory_compiled(self):
         with os_helper.temp_dir() as script_dir:
             script_name = _make_test_script(script_dir, '__main__',
                                             source=source)
-            py_compile.compile(script_name, doraise=True)
+            pyc_file = import_helper.make_legacy_pyc(script_name, allow_compile=True)
             os.remove(script_name)
-            pyc_file = import_helper.make_legacy_pyc(script_name)
             self._check_script(script_dir)
 
     def test_zipfile(self):
@@ -231,7 +229,8 @@ def test_zipfile_compiled(self):
         with os_helper.temp_dir() as script_dir:
             script_name = _make_test_script(script_dir, '__main__',
                                             source=source)
-            compiled_name = py_compile.compile(script_name, doraise=True)
+            compiled_name = script_name + 'c'
+            py_compile.compile(script_name, compiled_name, doraise=True)
             zip_name, run_name = make_zip_script(script_dir, 'test_zip', compiled_name)
             self._check_script(zip_name)
 
@@ -273,9 +272,8 @@ def test_package_compiled(self):
             make_pkg(pkg_dir)
             script_name = _make_test_script(pkg_dir, '__main__',
                                             source=source)
-            compiled_name = py_compile.compile(script_name, doraise=True)
+            pyc_file = import_helper.make_legacy_pyc(script_name, allow_compile=True)
             os.remove(script_name)
-            pyc_file = import_helper.make_legacy_pyc(script_name)
             launch_name = _make_launch_script(script_dir, 'launch', 'test_pkg')
             self._check_script(launch_name)
 
diff --git a/Lib/test/test_py_compile.py b/Lib/test/test_py_compile.py
index 64387296e84621..66de61930968e4 100644
--- a/Lib/test/test_py_compile.py
+++ b/Lib/test/test_py_compile.py
@@ -56,7 +56,10 @@ def setUp(self):
         self.directory = tempfile.mkdtemp(dir=os.getcwd())
         self.source_path = os.path.join(self.directory, '_test.py')
         self.pyc_path = self.source_path + 'c'
-        self.cache_path = importlib.util.cache_from_source(self.source_path)
+        try:
+            self.cache_path = importlib.util.cache_from_source(self.source_path)
+        except NotImplementedError:
+            self.cache_path = None
         self.cwd_drive = os.path.splitdrive(os.getcwd())[0]
         # In these tests we compute relative paths.  When using Windows, the
         # current working directory path and the 'self.source_path' might be
@@ -73,10 +76,31 @@ def tearDown(self):
         if self.cwd_drive:
             os.chdir(self.cwd_drive)
 
+    def assert_cache_path_exists(self, should_exist=True):
+        if self.cache_path:
+            if should_exist:
+                self.assertTrue(os.path.exists(self.cache_path))
+            else:
+                self.assertFalse(os.path.exists(self.cache_path))
+            return
+        cache_dir = os.path.join(self.directory, '__pycache__')
+        if not os.path.isdir(cache_dir):
+            if should_exist:
+                self.fail('no __pycache__ directory exists')
+            return
+        for f in os.listdir(cache_dir):
+            if f.startswith('_test.') and f.endswith('.pyc'):
+                if should_exist:
+                    return
+                self.fail(f'__pycache__/{f} was created')
+        else:
+            if should_exist:
+                self.fail('no __pycache__/_test.*.pyc file exists')
+
     def test_absolute_path(self):
         py_compile.compile(self.source_path, self.pyc_path)
         self.assertTrue(os.path.exists(self.pyc_path))
-        self.assertFalse(os.path.exists(self.cache_path))
+        self.assert_cache_path_exists(False)
 
     def test_do_not_overwrite_symlinks(self):
         # In the face of a cfile argument being a symlink, bail out.
@@ -98,22 +122,24 @@ def test_do_not_overwrite_nonregular_files(self):
         with self.assertRaises(FileExistsError):
             py_compile.compile(self.source_path, os.devnull)
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag is not None')
     def test_cache_path(self):
         py_compile.compile(self.source_path)
-        self.assertTrue(os.path.exists(self.cache_path))
+        self.assert_cache_path_exists(True)
 
     def test_cwd(self):
         with os_helper.change_cwd(self.directory):
             py_compile.compile(os.path.basename(self.source_path),
                                os.path.basename(self.pyc_path))
         self.assertTrue(os.path.exists(self.pyc_path))
-        self.assertFalse(os.path.exists(self.cache_path))
+        self.assert_cache_path_exists(False)
 
     def test_relative_path(self):
         py_compile.compile(os.path.relpath(self.source_path),
                            os.path.relpath(self.pyc_path))
         self.assertTrue(os.path.exists(self.pyc_path))
-        self.assertFalse(os.path.exists(self.cache_path))
+        self.assert_cache_path_exists(False)
 
     @os_helper.skip_if_dac_override
     @unittest.skipIf(os.name == 'nt',
@@ -136,14 +162,14 @@ def test_bad_coding(self):
                                   'tokenizedata',
                                   'bad_coding2.py')
         with support.captured_stderr():
-            self.assertIsNone(py_compile.compile(bad_coding, doraise=False))
-        self.assertFalse(os.path.exists(
-            importlib.util.cache_from_source(bad_coding)))
+            self.assertIsNone(py_compile.compile(bad_coding, self.pyc_path,
+                                                 doraise=False))
+        self.assertFalse(os.path.exists(self.pyc_path))
 
     def test_source_date_epoch(self):
         py_compile.compile(self.source_path, self.pyc_path)
         self.assertTrue(os.path.exists(self.pyc_path))
-        self.assertFalse(os.path.exists(self.cache_path))
+        self.assert_cache_path_exists(False)
         with open(self.pyc_path, 'rb') as fp:
             flags = importlib._bootstrap_external._classify_pyc(
                 fp.read(), 'test', {})
@@ -155,6 +181,8 @@ def test_source_date_epoch(self):
         self.assertEqual(flags, expected_flags)
 
     @unittest.skipIf(sys.flags.optimize > 0, 'test does not work with -O')
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag is not None')
     def test_double_dot_no_clobber(self):
         # http://bugs.python.org/issue22966
         # py_compile foo.bar.py -> __pycache__/foo.cpython-34.pyc
@@ -174,6 +202,8 @@ def test_double_dot_no_clobber(self):
         self.assertTrue(os.path.exists(cache_path))
         self.assertFalse(os.path.exists(pyc_path))
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag is not None')
     def test_optimization_path(self):
         # Specifying optimized bytecode should lead to a path reflecting that.
         self.assertIn('opt-2', py_compile.compile(self.source_path, optimize=2))
@@ -181,17 +211,19 @@ def test_optimization_path(self):
     def test_invalidation_mode(self):
         py_compile.compile(
             self.source_path,
+            self.pyc_path,
             invalidation_mode=py_compile.PycInvalidationMode.CHECKED_HASH,
         )
-        with open(self.cache_path, 'rb') as fp:
+        with open(self.pyc_path, 'rb') as fp:
             flags = importlib._bootstrap_external._classify_pyc(
                 fp.read(), 'test', {})
         self.assertEqual(flags, 0b11)
         py_compile.compile(
             self.source_path,
+            self.pyc_path,
             invalidation_mode=py_compile.PycInvalidationMode.UNCHECKED_HASH,
         )
-        with open(self.cache_path, 'rb') as fp:
+        with open(self.pyc_path, 'rb') as fp:
             flags = importlib._bootstrap_external._classify_pyc(
                 fp.read(), 'test', {})
         self.assertEqual(flags, 0b1)
@@ -201,11 +233,11 @@ def test_quiet(self):
                                   'tokenizedata',
                                   'bad_coding2.py')
         with support.captured_stderr() as stderr:
-            self.assertIsNone(py_compile.compile(bad_coding, doraise=False, quiet=2))
-            self.assertIsNone(py_compile.compile(bad_coding, doraise=True, quiet=2))
+            self.assertIsNone(py_compile.compile(bad_coding, self.pyc_path, doraise=False, quiet=2))
+            self.assertIsNone(py_compile.compile(bad_coding, self.pyc_path, doraise=True, quiet=2))
             self.assertEqual(stderr.getvalue(), '')
             with self.assertRaises(py_compile.PyCompileError):
-                py_compile.compile(bad_coding, doraise=True, quiet=1)
+                py_compile.compile(bad_coding, self.pyc_path, doraise=True, quiet=1)
 
 
 class PyCompileTestsWithSourceEpoch(PyCompileTestsBase,
@@ -227,8 +259,12 @@ class PyCompileCLITestCase(unittest.TestCase):
     def setUp(self):
         self.directory = tempfile.mkdtemp()
         self.source_path = os.path.join(self.directory, '_test.py')
-        self.cache_path = importlib.util.cache_from_source(self.source_path,
-                                optimization='' if __debug__ else 1)
+        try:
+            self.cache_path = importlib.util.cache_from_source(self.source_path,
+                                    optimization='' if __debug__ else 1)
+        except NotImplementedError:
+            # py_compile.main() assumes legacy pyc path if there is no cache_tag
+            self.cache_path = self.source_path + 'c'
         with open(self.source_path, 'w') as file:
             file.write('x = 123\n')
 
diff --git a/Lib/test/test_pydoc/test_pydoc.py b/Lib/test/test_pydoc/test_pydoc.py
index 0e113006cfa156..2e190d1b81be8e 100644
--- a/Lib/test/test_pydoc/test_pydoc.py
+++ b/Lib/test/test_pydoc/test_pydoc.py
@@ -1006,6 +1006,8 @@ def test_synopsis_sourceless(self):
         os = import_helper.import_fresh_module('os')
         expected = os.__doc__.splitlines()[0]
         filename = os.__spec__.cached
+        if not filename:
+            raise unittest.SkipTest('requires .pyc files')
         synopsis = pydoc.synopsis(filename)
 
         self.assertEqual(synopsis, expected)
@@ -1013,10 +1015,10 @@ def test_synopsis_sourceless(self):
     def test_synopsis_sourceless_empty_doc(self):
         with os_helper.temp_cwd() as test_dir:
             init_path = os.path.join(test_dir, 'foomod42.py')
-            cached_path = importlib.util.cache_from_source(init_path)
+            cached_path = init_path + 'c'
             with open(init_path, 'w') as fobj:
                 fobj.write("foo = 1")
-            py_compile.compile(init_path)
+            py_compile.compile(init_path, cached_path)
             synopsis = pydoc.synopsis(init_path, {})
             self.assertIsNone(synopsis)
             synopsis_cached = pydoc.synopsis(cached_path, {})
diff --git a/Lib/test/test_reprlib.py b/Lib/test/test_reprlib.py
index 22a55b57c076eb..5a95a05c6496a2 100644
--- a/Lib/test/test_reprlib.py
+++ b/Lib/test/test_reprlib.py
@@ -695,8 +695,11 @@ def _check_path_limitations(self, module_name):
         source_path_len += 2 * (len(self.longname) + 1)
         # a path separator + `module_name` + ".py"
         source_path_len += len(module_name) + 1 + len(".py")
-        cached_path_len = (source_path_len +
-            len(importlib.util.cache_from_source("x.py")) - len("x.py"))
+        try:
+            cached_path_len = (source_path_len +
+                len(importlib.util.cache_from_source("x.py")) - len("x.py"))
+        except NotImplementedError:
+            cached_path_len = source_path_len
         if os.name == 'nt' and cached_path_len >= 258:
             # Under Windows, the max path len is 260 including C's terminating
             # NUL character.
diff --git a/Lib/test/test_runpy.py b/Lib/test/test_runpy.py
index 254a009a69718b..9f3bc8973eb8ac 100644
--- a/Lib/test/test_runpy.py
+++ b/Lib/test/test_runpy.py
@@ -320,14 +320,16 @@ def create_ns(init_globals):
             self.check_code_execution(create_ns, expected_ns)
             importlib.invalidate_caches()
             __import__(mod_name)
-            os.remove(mod_fname)
             if not sys.dont_write_bytecode:
-                make_legacy_pyc(mod_fname)
+                make_legacy_pyc(mod_fname, allow_compile=True)
                 unload(mod_name)  # In case loader caches paths
+                os.remove(mod_fname)
                 importlib.invalidate_caches()
                 if verbose > 1: print("Running from compiled:", mod_name)
                 self._fix_ns_for_legacy_pyc(expected_ns, alter_sys)
                 self.check_code_execution(create_ns, expected_ns)
+            else:
+                os.remove(mod_fname)
         finally:
             self._del_pkg(pkg_dir)
         if verbose > 1: print("Module executed successfully")
@@ -360,14 +362,16 @@ def create_ns(init_globals):
             self.check_code_execution(create_ns, expected_ns)
             importlib.invalidate_caches()
             __import__(mod_name)
-            os.remove(mod_fname)
             if not sys.dont_write_bytecode:
-                make_legacy_pyc(mod_fname)
+                make_legacy_pyc(mod_fname, allow_compile=True)
                 unload(mod_name)  # In case loader caches paths
+                os.remove(mod_fname)
                 if verbose > 1: print("Running from compiled:", pkg_name)
                 importlib.invalidate_caches()
                 self._fix_ns_for_legacy_pyc(expected_ns, alter_sys)
                 self.check_code_execution(create_ns, expected_ns)
+            else:
+                os.remove(mod_fname)
         finally:
             self._del_pkg(pkg_dir)
         if verbose > 1: print("Package executed successfully")
@@ -420,7 +424,7 @@ def _check_relative_imports(self, depth, run_name=None):
             importlib.invalidate_caches()
             __import__(mod_name)
             os.remove(mod_fname)
-            if not sys.dont_write_bytecode:
+            if not sys.dont_write_bytecode and sys.implementation.cache_tag:
                 make_legacy_pyc(mod_fname)
                 unload(mod_name)  # In case the loader caches paths
                 if verbose > 1: print("Running from compiled:", mod_name)
@@ -676,6 +680,8 @@ def test_basic_script_no_suffix(self):
             self._check_script(script_name, "<run_path>", script_name,
                                script_name, expect_spec=False)
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag')
     def test_script_compiled(self):
         with temp_dir() as script_dir:
             mod_name = 'script'
@@ -696,12 +702,10 @@ def test_directory_compiled(self):
         with temp_dir() as script_dir:
             mod_name = '__main__'
             script_name = self._make_test_script(script_dir, mod_name)
-            compiled_name = py_compile.compile(script_name, doraise=True)
+            legacy_pyc = make_legacy_pyc(script_name, allow_compile=True)
             os.remove(script_name)
-            if not sys.dont_write_bytecode:
-                legacy_pyc = make_legacy_pyc(script_name)
-                self._check_script(script_dir, "<run_path>", legacy_pyc,
-                                   script_dir, mod_name=mod_name)
+            self._check_script(script_dir, "<run_path>", legacy_pyc,
+                               script_dir, mod_name=mod_name)
 
     def test_directory_error(self):
         with temp_dir() as script_dir:
@@ -722,7 +726,8 @@ def test_zipfile_compiled(self):
         with temp_dir() as script_dir:
             mod_name = '__main__'
             script_name = self._make_test_script(script_dir, mod_name)
-            compiled_name = py_compile.compile(script_name, doraise=True)
+            compiled_name = script_name + 'c'
+            py_compile.compile(script_name, compiled_name, doraise=True)
             zip_name, fname = make_zip_script(script_dir, 'test_zip',
                                               compiled_name)
             self._check_script(zip_name, "<run_path>", fname, zip_name,
diff --git a/Lib/test/test_zipimport.py b/Lib/test/test_zipimport.py
index dce3e1d9d38e7a..76cd85709a63af 100644
--- a/Lib/test/test_zipimport.py
+++ b/Lib/test/test_zipimport.py
@@ -60,7 +60,6 @@ def module_path_to_dotted_name(path):
 TEMP_ZIP = os.path.abspath("junk95142.zip")
 TEST_DATA_DIR = os.path.join(os.path.dirname(__file__), "zipimport_data")
 
-pyc_file = importlib.util.cache_from_source(TESTMOD + '.py')
 pyc_ext = '.pyc'
 
 
diff --git a/Lib/zipfile/__init__.py b/Lib/zipfile/__init__.py
index ac2332e58468a2..8234bf52d39c5f 100644
--- a/Lib/zipfile/__init__.py
+++ b/Lib/zipfile/__init__.py
@@ -2223,10 +2223,10 @@ def writepy(self, pathname, basename="", filterfunc=None):
                     basename = name
                 if self.debug:
                     print("Adding package in", pathname, "as", basename)
-                fname, arcname = self._get_codename(initname[0:-3], basename)
+                arcname, bytecode = self._get_code(initname[0:-3], basename)
                 if self.debug:
                     print("Adding", arcname)
-                self.write(fname, arcname)
+                self.writestr(arcname, bytecode)
                 dirlist = sorted(os.listdir(pathname))
                 dirlist.remove("__init__.py")
                 # Add all *.py files and package subdirectories
@@ -2243,11 +2243,10 @@ def writepy(self, pathname, basename="", filterfunc=None):
                             if self.debug:
                                 print('file %r skipped by filterfunc' % path)
                             continue
-                        fname, arcname = self._get_codename(path[0:-3],
-                                                            basename)
+                        arcname, bytecode = self._get_code(path[0:-3], basename)
                         if self.debug:
                             print("Adding", arcname)
-                        self.write(fname, arcname)
+                        self.writestr(arcname, bytecode)
             else:
                 # This is NOT a package directory, add its files at top level
                 if self.debug:
@@ -2260,101 +2259,58 @@ def writepy(self, pathname, basename="", filterfunc=None):
                             if self.debug:
                                 print('file %r skipped by filterfunc' % path)
                             continue
-                        fname, arcname = self._get_codename(path[0:-3],
-                                                            basename)
+                        arcname, bytecode = self._get_code(path[0:-3], basename)
                         if self.debug:
                             print("Adding", arcname)
-                        self.write(fname, arcname)
+                        self.writestr(arcname, bytecode)
         else:
             if pathname[-3:] != ".py":
                 raise RuntimeError(
                     'Files added with writepy() must end with ".py"')
-            fname, arcname = self._get_codename(pathname[0:-3], basename)
+            arcname, bytecode = self._get_code(pathname[0:-3], basename)
             if self.debug:
                 print("Adding file", arcname)
-            self.write(fname, arcname)
+            self.writestr(arcname, bytecode)
 
-    def _get_codename(self, pathname, basename):
-        """Return (filename, archivename) for the path.
+    def _get_code(self, pathname, basename):
+        """Return (arcname, bytecode) for the path.
 
-        Given a module name path, return the correct file path and
-        archive name, compiling if necessary.  For example, given
-        /python/lib/string, return (/python/lib/string.pyc, string).
+        Given a module name path, return the bytecode and archive
+        name.  For example, given /python/lib/string, return
+        ('string', b'<bytecode of string>').
         """
-        def _compile(file, optimize=-1):
-            import py_compile
-            if self.debug:
-                print("Compiling", file)
-            try:
-                py_compile.compile(file, doraise=True, optimize=optimize)
-            except py_compile.PyCompileError as err:
-                print(err.msg)
-                return False
-            return True
+        import importlib._bootstrap_external
+        import importlib.machinery
 
         file_py  = pathname + ".py"
         file_pyc = pathname + ".pyc"
-        pycache_opt0 = importlib.util.cache_from_source(file_py, optimization='')
-        pycache_opt1 = importlib.util.cache_from_source(file_py, optimization=1)
-        pycache_opt2 = importlib.util.cache_from_source(file_py, optimization=2)
-        if self._optimize == -1:
-            # legacy mode: use whatever file is present
-            if (os.path.isfile(file_pyc) and
-                  os.stat(file_pyc).st_mtime >= os.stat(file_py).st_mtime):
-                # Use .pyc file.
-                arcname = fname = file_pyc
-            elif (os.path.isfile(pycache_opt0) and
-                  os.stat(pycache_opt0).st_mtime >= os.stat(file_py).st_mtime):
-                # Use the __pycache__/*.pyc file, but write it to the legacy pyc
-                # file name in the archive.
-                fname = pycache_opt0
-                arcname = file_pyc
-            elif (os.path.isfile(pycache_opt1) and
-                  os.stat(pycache_opt1).st_mtime >= os.stat(file_py).st_mtime):
-                # Use the __pycache__/*.pyc file, but write it to the legacy pyc
-                # file name in the archive.
-                fname = pycache_opt1
-                arcname = file_pyc
-            elif (os.path.isfile(pycache_opt2) and
-                  os.stat(pycache_opt2).st_mtime >= os.stat(file_py).st_mtime):
-                # Use the __pycache__/*.pyc file, but write it to the legacy pyc
-                # file name in the archive.
-                fname = pycache_opt2
-                arcname = file_pyc
-            else:
-                # Compile py into PEP 3147 pyc file.
-                if _compile(file_py):
-                    if sys.flags.optimize == 0:
-                        fname = pycache_opt0
-                    elif sys.flags.optimize == 1:
-                        fname = pycache_opt1
-                    else:
-                        fname = pycache_opt2
-                    arcname = file_pyc
-                else:
-                    fname = arcname = file_py
+        archivename = os.path.split(file_pyc)[1]
+
+        loader = importlib.machinery.SourceFileLoader('<py_compile>', file_py)
+        source_bytes = loader.get_data(file_py)
+        try:
+            if self.debug:
+                print("Compiling", file_py)
+            code = loader.source_to_code(source_bytes, archivename)
+        except Exception as err:
+            # Historically, this function prints messages here rather than raising
+            # (see test_zipfile.test_write_filtered_python_package)
+            from py_compile import PyCompileError
+            print(PyCompileError(type(err), err, file_py).msg)
+
+            archivename = os.path.split(file_py)[1]
+            bytecode = source_bytes
         else:
-            # new mode: use given optimization level
-            if self._optimize == 0:
-                fname = pycache_opt0
-                arcname = file_pyc
-            else:
-                arcname = file_pyc
-                if self._optimize == 1:
-                    fname = pycache_opt1
-                elif self._optimize == 2:
-                    fname = pycache_opt2
-                else:
-                    msg = "invalid value for 'optimize': {!r}".format(self._optimize)
-                    raise ValueError(msg)
-            if not (os.path.isfile(fname) and
-                    os.stat(fname).st_mtime >= os.stat(file_py).st_mtime):
-                if not _compile(file_py, optimize=self._optimize):
-                    fname = arcname = file_py
-        archivename = os.path.split(arcname)[1]
+            # Historically this function has used timestamp comparisons, so we
+            # keep using it until someone makes that specific improvement.
+            source_stats = loader.path_stats(file_py)
+            bytecode = importlib._bootstrap_external._code_to_timestamp_pyc(
+                code, source_stats['mtime'], source_stats['size'])
+
         if basename:
             archivename = "%s/%s" % (basename, archivename)
-        return (fname, archivename)
+
+        return archivename, bytecode
 
 
 def main(args=None):
diff --git a/Modules/_testlimitedcapi/import.c b/Modules/_testlimitedcapi/import.c
index f85daee57d712e..f572212ba88b76 100644
--- a/Modules/_testlimitedcapi/import.c
+++ b/Modules/_testlimitedcapi/import.c
@@ -22,7 +22,7 @@ static PyObject *
 pyimport_getmagictag(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
 {
     const char *tag = PyImport_GetMagicTag();
-    return PyUnicode_FromString(tag);
+    return tag ? PyUnicode_FromString(tag) : Py_NewRef(Py_None);
 }
 
 
diff --git a/PC/pyconfig.h b/PC/pyconfig.h
index a126fca6f5aafb..c9c1015e7a9bc9 100644
--- a/PC/pyconfig.h
+++ b/PC/pyconfig.h
@@ -765,4 +765,7 @@ Py_NO_ENABLE_SHARED to find out.  Also support MS_NO_COREDLL for b/w compat */
 // Truncate the thread name to 32766 characters.
 #define _PYTHREAD_NAME_MAXLEN 32766
 
+// DO NOT MERGE
+#define _PY_DISABLE_SYS_CACHE_TAG
+
 #endif /* !Py_CONFIG_H */
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 94eb3164ecad58..111e3ae2650752 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -3572,7 +3572,11 @@ make_version_info(PyThreadState *tstate)
 const char *_PySys_ImplName = NAME;
 #define MAJOR Py_STRINGIFY(PY_MAJOR_VERSION)
 #define MINOR Py_STRINGIFY(PY_MINOR_VERSION)
+#ifdef _PY_DISABLE_SYS_CACHE_TAG
+#define TAG NULL
+#else
 #define TAG NAME "-" MAJOR MINOR
+#endif
 const char *_PySys_ImplCacheTag = TAG;
 #undef NAME
 #undef MAJOR
@@ -3599,9 +3603,12 @@ make_impl_info(PyObject *version_info)
     if (res < 0)
         goto error;
 
-    value = PyUnicode_FromString(_PySys_ImplCacheTag);
-    if (value == NULL)
+    value = _PySys_ImplCacheTag
+        ? PyUnicode_FromString(_PySys_ImplCacheTag)
+        : Py_NewRef(Py_None);
+    if (value == NULL) {
         goto error;
+    }
     res = PyDict_SetItemString(impl_info, "cache_tag", value);
     Py_DECREF(value);
     if (res < 0)

From d831daa387592411b27b327a79180d34ef9c659f Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Tue, 27 Jan 2026 23:42:20 +0000
Subject: [PATCH 03/29] Respect allow_compile flag

---
 Lib/test/support/import_helper.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Lib/test/support/import_helper.py b/Lib/test/support/import_helper.py
index f1642e51daf815..a320ba70f72f81 100644
--- a/Lib/test/support/import_helper.py
+++ b/Lib/test/support/import_helper.py
@@ -71,6 +71,8 @@ def make_legacy_pyc(source, allow_compile=False):
     try:
         pyc_file = importlib.util.cache_from_source(source)
     except NotImplementedError:
+        if not allow_compile:
+            raise
         py_compile.compile(source, legacy_pyc, doraise=True)
     else:
         shutil.move(pyc_file, legacy_pyc)

From 1fd6a3097fc4c10b6f3188dff8ab25022c23afa6 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Tue, 27 Jan 2026 23:44:08 +0000
Subject: [PATCH 04/29] Remove unused import

---
 Lib/test/test_argparse.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index d2e9903f897bed..78f02f70b9f0fc 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -7,7 +7,6 @@
 import io
 import operator
 import os
-import py_compile
 import shutil
 import stat
 import sys

From 601e5375ba15b24f2b6ab8c9a493ba4d422ceec4 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Tue, 27 Jan 2026 23:55:41 +0000
Subject: [PATCH 05/29] Fix NEWS reference

---
 .../next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst   | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst b/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst
index fa6b0c6e505cdb..687a469f170cc8 100644
--- a/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst
+++ b/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst
@@ -1,4 +1,4 @@
 Enables patching in a ``_PY_DISABLE_SYS_CACHE_TAG`` preprocessor definition
-when building to force :data:`sys.implementation.cache_tag` to ``None``.
+when building to force :attr:`sys.implementation.cache_tag` to ``None``.
 This has the effect of completely disabling automatic creation and reading
 of ``.pyc`` files.

From df97ccdb7fbff3e101bbe7192de5c81379265154 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Wed, 28 Jan 2026 13:49:27 +0000
Subject: [PATCH 06/29] Make TAG definable instaed

---
 Lib/test/support/import_helper.py | 5 ++---
 PC/pyconfig.h                     | 3 ---
 Python/sysmodule.c                | 7 ++++---
 3 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/Lib/test/support/import_helper.py b/Lib/test/support/import_helper.py
index a320ba70f72f81..093de6a82d8ca7 100644
--- a/Lib/test/support/import_helper.py
+++ b/Lib/test/support/import_helper.py
@@ -70,12 +70,11 @@ def make_legacy_pyc(source, allow_compile=False):
     legacy_pyc = source + 'c'
     try:
         pyc_file = importlib.util.cache_from_source(source)
-    except NotImplementedError:
+        shutil.move(pyc_file, legacy_pyc)
+    except (FileNotFoundError, NotImplementedError):
         if not allow_compile:
             raise
         py_compile.compile(source, legacy_pyc, doraise=True)
-    else:
-        shutil.move(pyc_file, legacy_pyc)
     return legacy_pyc
 
 
diff --git a/PC/pyconfig.h b/PC/pyconfig.h
index c9c1015e7a9bc9..a126fca6f5aafb 100644
--- a/PC/pyconfig.h
+++ b/PC/pyconfig.h
@@ -765,7 +765,4 @@ Py_NO_ENABLE_SHARED to find out.  Also support MS_NO_COREDLL for b/w compat */
 // Truncate the thread name to 32766 characters.
 #define _PYTHREAD_NAME_MAXLEN 32766
 
-// DO NOT MERGE
-#define _PY_DISABLE_SYS_CACHE_TAG
-
 #endif /* !Py_CONFIG_H */
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 111e3ae2650752..87872e61da1423 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -1,3 +1,6 @@
+// DO NOT MERGE
+#define TAG NULL
+
 
 /* System module */
 
@@ -3572,9 +3575,7 @@ make_version_info(PyThreadState *tstate)
 const char *_PySys_ImplName = NAME;
 #define MAJOR Py_STRINGIFY(PY_MAJOR_VERSION)
 #define MINOR Py_STRINGIFY(PY_MINOR_VERSION)
-#ifdef _PY_DISABLE_SYS_CACHE_TAG
-#define TAG NULL
-#else
+#ifndef TAG
 #define TAG NAME "-" MAJOR MINOR
 #endif
 const char *_PySys_ImplCacheTag = TAG;

From cbd1e15c69781d85053c203a9abbb89e8af20eea Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Wed, 28 Jan 2026 15:18:21 +0000
Subject: [PATCH 07/29] Suppress pip compile without cache tag

---
 Lib/ensurepip/__init__.py                                 | 2 ++
 .../Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst  | 8 ++++----
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/Lib/ensurepip/__init__.py b/Lib/ensurepip/__init__.py
index f9f905f46ff09a..db8f0ac630edc0 100644
--- a/Lib/ensurepip/__init__.py
+++ b/Lib/ensurepip/__init__.py
@@ -177,6 +177,8 @@ def _bootstrap(*, root=None, upgrade=False, user=False,
             args += ["--user"]
         if verbosity:
             args += ["-" + "v" * verbosity]
+        if sys.implementation.cache_tag is None:
+            args += ["--no-compile"]
 
         return _run_pip([*args, "pip"], [os.fsdecode(tmp_wheel_path)])
 
diff --git a/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst b/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst
index 687a469f170cc8..63a69d3115bcf4 100644
--- a/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst
+++ b/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst
@@ -1,4 +1,4 @@
-Enables patching in a ``_PY_DISABLE_SYS_CACHE_TAG`` preprocessor definition
-when building to force :attr:`sys.implementation.cache_tag` to ``None``.
-This has the effect of completely disabling automatic creation and reading
-of ``.pyc`` files.
+Enables patching the ``TAG`` preprocessor definition to override
+:attr:`sys.implementation.cache_tag` at build time. Setting it to ``NULL``
+has the effect of completely disabling automatic creation and use of
+``.pyc`` files.

From 0429aa297c7d28abac3be8e5ea41439991e41a31 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Wed, 28 Jan 2026 15:27:57 +0000
Subject: [PATCH 08/29] Test ensurepip for new flag

---
 Lib/test/test_ensurepip.py | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/Lib/test/test_ensurepip.py b/Lib/test/test_ensurepip.py
index f6743d57ca28dd..c62b340f6a340f 100644
--- a/Lib/test/test_ensurepip.py
+++ b/Lib/test/test_ensurepip.py
@@ -12,6 +12,12 @@
 import ensurepip._uninstall
 
 
+if sys.implementation.cache_tag is None:
+    COMPILE_OPT = ["--no-compile"]
+else:
+    COMPILE_OPT = []
+
+
 class TestPackages(unittest.TestCase):
     def touch(self, directory, filename):
         fullname = os.path.join(directory, filename)
@@ -85,7 +91,7 @@ def test_basic_bootstrapping(self):
         self.run_pip.assert_called_once_with(
             [
                 "install", "--no-cache-dir", "--no-index", "--find-links",
-                unittest.mock.ANY, "pip",
+                unittest.mock.ANY, *COMPILE_OPT, "pip",
             ],
             unittest.mock.ANY,
         )
@@ -99,7 +105,7 @@ def test_bootstrapping_with_root(self):
         self.run_pip.assert_called_once_with(
             [
                 "install", "--no-cache-dir", "--no-index", "--find-links",
-                unittest.mock.ANY, "--root", "/foo/bar/",
+                unittest.mock.ANY, "--root", "/foo/bar/", *COMPILE_OPT,
                 "pip",
             ],
             unittest.mock.ANY,
@@ -111,7 +117,7 @@ def test_bootstrapping_with_user(self):
         self.run_pip.assert_called_once_with(
             [
                 "install", "--no-cache-dir", "--no-index", "--find-links",
-                unittest.mock.ANY, "--user", "pip",
+                unittest.mock.ANY, "--user", *COMPILE_OPT, "pip",
             ],
             unittest.mock.ANY,
         )
@@ -122,7 +128,7 @@ def test_bootstrapping_with_upgrade(self):
         self.run_pip.assert_called_once_with(
             [
                 "install", "--no-cache-dir", "--no-index", "--find-links",
-                unittest.mock.ANY, "--upgrade", "pip",
+                unittest.mock.ANY, "--upgrade", *COMPILE_OPT, "pip",
             ],
             unittest.mock.ANY,
         )
@@ -133,7 +139,7 @@ def test_bootstrapping_with_verbosity_1(self):
         self.run_pip.assert_called_once_with(
             [
                 "install", "--no-cache-dir", "--no-index", "--find-links",
-                unittest.mock.ANY, "-v", "pip",
+                unittest.mock.ANY, "-v", *COMPILE_OPT, "pip",
             ],
             unittest.mock.ANY,
         )
@@ -144,7 +150,7 @@ def test_bootstrapping_with_verbosity_2(self):
         self.run_pip.assert_called_once_with(
             [
                 "install", "--no-cache-dir", "--no-index", "--find-links",
-                unittest.mock.ANY, "-vv", "pip",
+                unittest.mock.ANY, "-vv", *COMPILE_OPT, "pip",
             ],
             unittest.mock.ANY,
         )
@@ -155,7 +161,7 @@ def test_bootstrapping_with_verbosity_3(self):
         self.run_pip.assert_called_once_with(
             [
                 "install", "--no-cache-dir", "--no-index", "--find-links",
-                unittest.mock.ANY, "-vvv", "pip",
+                unittest.mock.ANY, "-vvv", *COMPILE_OPT, "pip",
             ],
             unittest.mock.ANY,
         )
@@ -312,7 +318,7 @@ def test_basic_bootstrapping(self):
         self.run_pip.assert_called_once_with(
             [
                 "install", "--no-cache-dir", "--no-index", "--find-links",
-                unittest.mock.ANY, "pip",
+                unittest.mock.ANY, *COMPILE_OPT, "pip",
             ],
             unittest.mock.ANY,
         )

From b8e633569e5163f71cc11eb54d7a87c1a947c767 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Wed, 28 Jan 2026 15:41:29 +0000
Subject: [PATCH 09/29] Fix NEWS and suppress pip compilation for testing

---
 .github/workflows/build.yml                                 | 1 +
 .../Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst    | 6 +++---
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index e7f7aa5172e082..11bf6def9ff9e5 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -26,6 +26,7 @@ concurrency:
 
 env:
   FORCE_COLOR: 1
+  PIP_NO_COMPILE: 1
 
 jobs:
   build-context:
diff --git a/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst b/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst
index 63a69d3115bcf4..3965a5036a8e57 100644
--- a/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst
+++ b/Misc/NEWS.d/next/Build/2026-01-27-23-39-26.gh-issue-144278.tejFwL.rst
@@ -1,4 +1,4 @@
 Enables patching the ``TAG`` preprocessor definition to override
-:attr:`sys.implementation.cache_tag` at build time. Setting it to ``NULL``
-has the effect of completely disabling automatic creation and use of
-``.pyc`` files.
+:data:`sys.implementation.cache_tag <sys.implementation>` at build time.
+Setting it to ``NULL`` has the effect of completely disabling automatic
+creation and use of ``.pyc`` files.

From 142f0098f98e702ec3a620cf16bb20beb3b5bc50 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Wed, 28 Jan 2026 16:26:13 +0000
Subject: [PATCH 10/29] Set env on each step

---
 .github/workflows/build.yml           | 5 ++++-
 .github/workflows/reusable-cifuzz.yml | 3 +++
 .github/workflows/reusable-docs.yml   | 3 +++
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 11bf6def9ff9e5..f585e897e96df9 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -26,7 +26,6 @@ concurrency:
 
 env:
   FORCE_COLOR: 1
-  PIP_NO_COMPILE: 1
 
 jobs:
   build-context:
@@ -431,6 +430,8 @@ jobs:
     env:
       OPENSSL_VER: 3.0.18
       PYTHONSTRICTEXTENSIONBUILD: 1
+      # DO NOT MERGE
+      PIP_NO_COMPILE: 1
     steps:
     - uses: actions/checkout@v6
       with:
@@ -543,6 +544,8 @@ jobs:
       OPENSSL_VER: 3.0.18
       PYTHONSTRICTEXTENSIONBUILD: 1
       ASAN_OPTIONS: detect_leaks=0:allocator_may_return_null=1:handle_segv=0
+      # DO NOT MERGE
+      PIP_NO_COMPILE: 1
     steps:
     - uses: actions/checkout@v6
       with:
diff --git a/.github/workflows/reusable-cifuzz.yml b/.github/workflows/reusable-cifuzz.yml
index 1986f5fb2cc640..a28e549839262c 100644
--- a/.github/workflows/reusable-cifuzz.yml
+++ b/.github/workflows/reusable-cifuzz.yml
@@ -18,6 +18,9 @@ jobs:
     name: ${{ inputs.oss-fuzz-project-name }} (${{ inputs.sanitizer }})
     runs-on: ubuntu-latest
     timeout-minutes: 60
+    env:
+      # DO NOT MERGE
+      PIP_NO_COMPILE: 1
     steps:
       - name: Build fuzzers (${{ inputs.sanitizer }})
         id: build
diff --git a/.github/workflows/reusable-docs.yml b/.github/workflows/reusable-docs.yml
index fc68c040fca059..5c5623ef02726d 100644
--- a/.github/workflows/reusable-docs.yml
+++ b/.github/workflows/reusable-docs.yml
@@ -81,6 +81,9 @@ jobs:
     name: 'Doctest'
     runs-on: ubuntu-24.04
     timeout-minutes: 60
+    env:
+      # DO NOT MERGE
+      PIP_NO_COMPILE: 1
     steps:
     - uses: actions/checkout@v6
       with:

From 2b0f29cd38470c35f1014fdbdf9a1661157d9501 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Wed, 28 Jan 2026 16:59:09 +0000
Subject: [PATCH 11/29] Make compileall silently ignore missing cache_tag

---
 .github/workflows/build.yml           | 4 ----
 .github/workflows/reusable-cifuzz.yml | 3 ---
 .github/workflows/reusable-docs.yml   | 3 ---
 Lib/compileall.py                     | 9 +++++++++
 4 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index f585e897e96df9..e7f7aa5172e082 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -430,8 +430,6 @@ jobs:
     env:
       OPENSSL_VER: 3.0.18
       PYTHONSTRICTEXTENSIONBUILD: 1
-      # DO NOT MERGE
-      PIP_NO_COMPILE: 1
     steps:
     - uses: actions/checkout@v6
       with:
@@ -544,8 +542,6 @@ jobs:
       OPENSSL_VER: 3.0.18
       PYTHONSTRICTEXTENSIONBUILD: 1
       ASAN_OPTIONS: detect_leaks=0:allocator_may_return_null=1:handle_segv=0
-      # DO NOT MERGE
-      PIP_NO_COMPILE: 1
     steps:
     - uses: actions/checkout@v6
       with:
diff --git a/.github/workflows/reusable-cifuzz.yml b/.github/workflows/reusable-cifuzz.yml
index a28e549839262c..1986f5fb2cc640 100644
--- a/.github/workflows/reusable-cifuzz.yml
+++ b/.github/workflows/reusable-cifuzz.yml
@@ -18,9 +18,6 @@ jobs:
     name: ${{ inputs.oss-fuzz-project-name }} (${{ inputs.sanitizer }})
     runs-on: ubuntu-latest
     timeout-minutes: 60
-    env:
-      # DO NOT MERGE
-      PIP_NO_COMPILE: 1
     steps:
       - name: Build fuzzers (${{ inputs.sanitizer }})
         id: build
diff --git a/.github/workflows/reusable-docs.yml b/.github/workflows/reusable-docs.yml
index 5c5623ef02726d..fc68c040fca059 100644
--- a/.github/workflows/reusable-docs.yml
+++ b/.github/workflows/reusable-docs.yml
@@ -81,9 +81,6 @@ jobs:
     name: 'Doctest'
     runs-on: ubuntu-24.04
     timeout-minutes: 60
-    env:
-      # DO NOT MERGE
-      PIP_NO_COMPILE: 1
     steps:
     - uses: actions/checkout@v6
       with:
diff --git a/Lib/compileall.py b/Lib/compileall.py
index 9519a5ac16f024..0d8edb3c107712 100644
--- a/Lib/compileall.py
+++ b/Lib/compileall.py
@@ -165,6 +165,15 @@ def compile_file(fullname, ddir=None, force=False, rx=None, quiet=0,
     stripdir = os.fspath(stripdir) if stripdir is not None else None
     name = os.path.basename(fullname)
 
+    # Without a cache_tag, we can only create legacy .pyc files. None of our
+    # callers seem to expect this, so the best we can do is silently succeed
+    # without creating anything.
+    if not legacy and sys.implementation.cache_tag is None:
+        if not quiet:
+            print("No cache tag is available to generate .pyc path for {!r}"
+                    .format(fullname))
+        return True
+
     dfile = None
 
     if ddir is not None:

From 29502a7f7321f0bd4701c7709a7e024c313cb2de Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Wed, 28 Jan 2026 17:59:26 +0000
Subject: [PATCH 12/29] Fail instead

---
 Lib/compileall.py | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/Lib/compileall.py b/Lib/compileall.py
index 0d8edb3c107712..39eadcb032eb81 100644
--- a/Lib/compileall.py
+++ b/Lib/compileall.py
@@ -166,13 +166,12 @@ def compile_file(fullname, ddir=None, force=False, rx=None, quiet=0,
     name = os.path.basename(fullname)
 
     # Without a cache_tag, we can only create legacy .pyc files. None of our
-    # callers seem to expect this, so the best we can do is silently succeed
-    # without creating anything.
+    # callers seem to expect this, so the best we can do is fail without raising
     if not legacy and sys.implementation.cache_tag is None:
         if not quiet:
             print("No cache tag is available to generate .pyc path for {!r}"
                     .format(fullname))
-        return True
+        return False
 
     dfile = None
 

From bd2731a12eeceb8c6777163a53879435d6fc1138 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Wed, 28 Jan 2026 22:34:31 +0000
Subject: [PATCH 13/29] Test without null tag

---
 Python/sysmodule.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 87872e61da1423..53ec6378ce89b2 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -1,5 +1,5 @@
 // DO NOT MERGE
-#define TAG NULL
+//#define TAG NULL
 
 
 /* System module */

From c51bcca2b2f05019143b52a79b6f6e7ec51f43d6 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Wed, 28 Jan 2026 22:35:19 +0000
Subject: [PATCH 14/29] Avoid unnecessary .format

---
 Lib/compileall.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Lib/compileall.py b/Lib/compileall.py
index 39eadcb032eb81..c452aed135838f 100644
--- a/Lib/compileall.py
+++ b/Lib/compileall.py
@@ -169,8 +169,8 @@ def compile_file(fullname, ddir=None, force=False, rx=None, quiet=0,
     # callers seem to expect this, so the best we can do is fail without raising
     if not legacy and sys.implementation.cache_tag is None:
         if not quiet:
-            print("No cache tag is available to generate .pyc path for {!r}"
-                    .format(fullname))
+            print("No cache tag is available to generate .pyc path for",
+                  repr(fullname))
         return False
 
     dfile = None

From 11546af67995623e75003522fb649601f882b445 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Wed, 28 Jan 2026 22:56:45 +0000
Subject: [PATCH 15/29] Make TAG NULL again

---
 Python/sysmodule.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 53ec6378ce89b2..87872e61da1423 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -1,5 +1,5 @@
 // DO NOT MERGE
-//#define TAG NULL
+#define TAG NULL
 
 
 /* System module */

From 20edcd4239783c6c4e3ca0330130edaf056c5e44 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Thu, 29 Jan 2026 00:11:30 +0000
Subject: [PATCH 16/29] Potential fast path

---
 Lib/importlib/_bootstrap_external.py | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/Lib/importlib/_bootstrap_external.py b/Lib/importlib/_bootstrap_external.py
index b576ceb1ce9f6e..1c05b40f4e8e35 100644
--- a/Lib/importlib/_bootstrap_external.py
+++ b/Lib/importlib/_bootstrap_external.py
@@ -820,10 +820,13 @@ def get_code(self, fullname):
         source_hash = None
         hash_based = False
         check_source = True
-        try:
-            bytecode_path = cache_from_source(source_path)
-        except NotImplementedError:
-            bytecode_path = None
+        bytecode_path = None
+        # TESTING: Potential fast path
+        if sys.implementation.cache_tag is not None:
+            try:
+                bytecode_path = cache_from_source(source_path)
+            except NotImplementedError:
+                pass
         else:
             try:
                 st = self.path_stats(source_path)

From 4ab201b23036548365afc64c0028fa510b6c337b Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Thu, 29 Jan 2026 00:12:06 +0000
Subject: [PATCH 17/29] Fix fast path

---
 Lib/importlib/_bootstrap_external.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Lib/importlib/_bootstrap_external.py b/Lib/importlib/_bootstrap_external.py
index 1c05b40f4e8e35..576de6596bda96 100644
--- a/Lib/importlib/_bootstrap_external.py
+++ b/Lib/importlib/_bootstrap_external.py
@@ -827,7 +827,7 @@ def get_code(self, fullname):
                 bytecode_path = cache_from_source(source_path)
             except NotImplementedError:
                 pass
-        else:
+        if bytecode_path:
             try:
                 st = self.path_stats(source_path)
             except OSError:

From d60a6be2f364d8d782251b1a0ac63122563e290a Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Thu, 29 Jan 2026 17:34:31 +0000
Subject: [PATCH 18/29] Check running tests without existing .pyc files

---
 .github/workflows/reusable-windows.yml | 3 +++
 Python/sysmodule.c                     | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/.github/workflows/reusable-windows.yml b/.github/workflows/reusable-windows.yml
index 82ea819867ef6d..e7449251be6ac4 100644
--- a/.github/workflows/reusable-windows.yml
+++ b/.github/workflows/reusable-windows.yml
@@ -41,6 +41,9 @@ jobs:
       shell: bash
     - name: Display build info
       run: .\\python.bat -m test.pythoninfo
+    - name: DO NOT MERGE cleanup pyc
+      run: dir Lib/*.pyc -r | del
+      shell: powershell
     - name: Tests
       run: >-
         .\\PCbuild\\rt.bat
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 87872e61da1423..53ec6378ce89b2 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -1,5 +1,5 @@
 // DO NOT MERGE
-#define TAG NULL
+//#define TAG NULL
 
 
 /* System module */

From 97d34e261255df4cc7abf14f01093a4add05f406 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Thu, 29 Jan 2026 20:10:41 +0000
Subject: [PATCH 19/29] Try interning name and path eagerly

---
 .github/workflows/reusable-windows.yml |  3 ---
 Lib/importlib/_bootstrap_external.py   | 17 +++++++----------
 2 files changed, 7 insertions(+), 13 deletions(-)

diff --git a/.github/workflows/reusable-windows.yml b/.github/workflows/reusable-windows.yml
index e7449251be6ac4..82ea819867ef6d 100644
--- a/.github/workflows/reusable-windows.yml
+++ b/.github/workflows/reusable-windows.yml
@@ -41,9 +41,6 @@ jobs:
       shell: bash
     - name: Display build info
       run: .\\python.bat -m test.pythoninfo
-    - name: DO NOT MERGE cleanup pyc
-      run: dir Lib/*.pyc -r | del
-      shell: powershell
     - name: Tests
       run: >-
         .\\PCbuild\\rt.bat
diff --git a/Lib/importlib/_bootstrap_external.py b/Lib/importlib/_bootstrap_external.py
index 576de6596bda96..20f93ce9956b36 100644
--- a/Lib/importlib/_bootstrap_external.py
+++ b/Lib/importlib/_bootstrap_external.py
@@ -820,14 +820,11 @@ def get_code(self, fullname):
         source_hash = None
         hash_based = False
         check_source = True
-        bytecode_path = None
-        # TESTING: Potential fast path
-        if sys.implementation.cache_tag is not None:
-            try:
-                bytecode_path = cache_from_source(source_path)
-            except NotImplementedError:
-                pass
-        if bytecode_path:
+        try:
+            bytecode_path = cache_from_source(source_path)
+        except NotImplementedError:
+            bytecode_path = None
+        else:
             try:
                 st = self.path_stats(source_path)
             except OSError:
@@ -904,8 +901,8 @@ class FileLoader:
     def __init__(self, fullname, path):
         """Cache the module name and the path to the file found by the
         finder."""
-        self.name = fullname
-        self.path = path
+        self.name = sys.intern(fullname)
+        self.path = sys.intern(path)
 
     def __eq__(self, other):
         return (self.__class__ == other.__class__ and

From 2f8fa9ebaea903ef70264db0549d9e48bb51bf93 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Thu, 29 Jan 2026 20:20:03 +0000
Subject: [PATCH 20/29] Don't intern name tuple

---
 Lib/importlib/_bootstrap_external.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Lib/importlib/_bootstrap_external.py b/Lib/importlib/_bootstrap_external.py
index 20f93ce9956b36..42aa58d3709ac8 100644
--- a/Lib/importlib/_bootstrap_external.py
+++ b/Lib/importlib/_bootstrap_external.py
@@ -901,7 +901,7 @@ class FileLoader:
     def __init__(self, fullname, path):
         """Cache the module name and the path to the file found by the
         finder."""
-        self.name = sys.intern(fullname)
+        self.name = fullname
         self.path = sys.intern(path)
 
     def __eq__(self, other):

From 003e6c13e96fe7b875a20377eb45c6e77b8b0df7 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Thu, 29 Jan 2026 20:38:09 +0000
Subject: [PATCH 21/29] Fix flag and only intern strings

---
 Lib/importlib/_bootstrap_external.py | 5 ++++-
 Python/sysmodule.c                   | 2 +-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/Lib/importlib/_bootstrap_external.py b/Lib/importlib/_bootstrap_external.py
index 42aa58d3709ac8..851e4fec9a21a5 100644
--- a/Lib/importlib/_bootstrap_external.py
+++ b/Lib/importlib/_bootstrap_external.py
@@ -902,7 +902,10 @@ def __init__(self, fullname, path):
         """Cache the module name and the path to the file found by the
         finder."""
         self.name = fullname
-        self.path = sys.intern(path)
+        if isinstance(path, str):
+            self.path = sys.intern(path)
+        else:
+            sys.path = path
 
     def __eq__(self, other):
         return (self.__class__ == other.__class__ and
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 53ec6378ce89b2..87872e61da1423 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -1,5 +1,5 @@
 // DO NOT MERGE
-//#define TAG NULL
+#define TAG NULL
 
 
 /* System module */

From 29666424eae36cd0f8f838d469ac30395b30238b Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Thu, 29 Jan 2026 20:41:28 +0000
Subject: [PATCH 22/29] Spelling

---
 Lib/importlib/_bootstrap_external.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Lib/importlib/_bootstrap_external.py b/Lib/importlib/_bootstrap_external.py
index 851e4fec9a21a5..c9fa9630ccfe63 100644
--- a/Lib/importlib/_bootstrap_external.py
+++ b/Lib/importlib/_bootstrap_external.py
@@ -905,7 +905,7 @@ def __init__(self, fullname, path):
         if isinstance(path, str):
             self.path = sys.intern(path)
         else:
-            sys.path = path
+            self.path = path
 
     def __eq__(self, other):
         return (self.__class__ == other.__class__ and

From 2d5912e51e6fa9a3fba4e753bebea45680b702ba Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Fri, 30 Jan 2026 01:09:40 +0000
Subject: [PATCH 23/29] Unconditionally skip .pyc creation to see if that's the
 cause

---
 Lib/importlib/_bootstrap_external.py | 7 +++----
 Python/sysmodule.c                   | 9 ++++++++-
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/Lib/importlib/_bootstrap_external.py b/Lib/importlib/_bootstrap_external.py
index c9fa9630ccfe63..b95c9e09bd9bc2 100644
--- a/Lib/importlib/_bootstrap_external.py
+++ b/Lib/importlib/_bootstrap_external.py
@@ -821,6 +821,8 @@ def get_code(self, fullname):
         hash_based = False
         check_source = True
         try:
+            # DO NOT MERGE - just testing unconditionally skipping this path
+            raise NotImplementedError()
             bytecode_path = cache_from_source(source_path)
         except NotImplementedError:
             bytecode_path = None
@@ -902,10 +904,7 @@ def __init__(self, fullname, path):
         """Cache the module name and the path to the file found by the
         finder."""
         self.name = fullname
-        if isinstance(path, str):
-            self.path = sys.intern(path)
-        else:
-            self.path = path
+        self.path = path
 
     def __eq__(self, other):
         return (self.__class__ == other.__class__ and
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 87872e61da1423..5c819b8d6e0075 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -1,5 +1,5 @@
 // DO NOT MERGE
-#define TAG NULL
+//#define TAG NULL
 
 
 /* System module */
@@ -3604,6 +3604,13 @@ make_impl_info(PyObject *version_info)
     if (res < 0)
         goto error;
 
+// DO NOT MERGE
+#ifdef MS_WINDOWS
+    if (GetEnvironmentVariableW(L"PYNOCACHETAG", NULL, 0)) {
+        _PySys_ImplCacheTag = NULL;
+    }
+#endif
+
     value = _PySys_ImplCacheTag
         ? PyUnicode_FromString(_PySys_ImplCacheTag)
         : Py_NewRef(Py_None);

From fcca3f9232421bce010e84ea33b63a8bed091ce7 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Fri, 30 Jan 2026 10:51:32 +0000
Subject: [PATCH 24/29] Test skipping pyc at lower level

---
 Lib/importlib/_bootstrap_external.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/Lib/importlib/_bootstrap_external.py b/Lib/importlib/_bootstrap_external.py
index b95c9e09bd9bc2..1ea606bc27e91a 100644
--- a/Lib/importlib/_bootstrap_external.py
+++ b/Lib/importlib/_bootstrap_external.py
@@ -821,13 +821,13 @@ def get_code(self, fullname):
         hash_based = False
         check_source = True
         try:
-            # DO NOT MERGE - just testing unconditionally skipping this path
-            raise NotImplementedError()
             bytecode_path = cache_from_source(source_path)
         except NotImplementedError:
             bytecode_path = None
         else:
             try:
+                # DO NOT MERGE - just testing unconditionally skipping this path
+                raise OSError()
                 st = self.path_stats(source_path)
             except OSError:
                 pass
@@ -889,6 +889,8 @@ def get_code(self, fullname):
                 data = _code_to_timestamp_pyc(code_object, source_mtime,
                                               len(source_bytes))
             try:
+                # DO NOT MERGE - just testing unconditionally skipping this path
+                raise NotImplementedError()
                 self._cache_bytecode(source_path, bytecode_path, data)
             except NotImplementedError:
                 pass

From 6a588e88dec406629aaf67682ecaf0164ec160d3 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Fri, 30 Jan 2026 11:01:20 +0000
Subject: [PATCH 25/29] Just skip pyc writing

---
 Lib/importlib/_bootstrap_external.py | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/Lib/importlib/_bootstrap_external.py b/Lib/importlib/_bootstrap_external.py
index 1ea606bc27e91a..75490acd857d1e 100644
--- a/Lib/importlib/_bootstrap_external.py
+++ b/Lib/importlib/_bootstrap_external.py
@@ -826,8 +826,6 @@ def get_code(self, fullname):
             bytecode_path = None
         else:
             try:
-                # DO NOT MERGE - just testing unconditionally skipping this path
-                raise OSError()
                 st = self.path_stats(source_path)
             except OSError:
                 pass
@@ -889,8 +887,6 @@ def get_code(self, fullname):
                 data = _code_to_timestamp_pyc(code_object, source_mtime,
                                               len(source_bytes))
             try:
-                # DO NOT MERGE - just testing unconditionally skipping this path
-                raise NotImplementedError()
                 self._cache_bytecode(source_path, bytecode_path, data)
             except NotImplementedError:
                 pass
@@ -951,6 +947,8 @@ def _cache_bytecode(self, source_path, bytecode_path, data):
 
     def set_data(self, path, data, *, _mode=0o666):
         """Write bytes data to a file."""
+        # DO NOT MERGE - just testing unconditionally skipping this path
+        return
         parent, filename = _path_split(path)
         path_parts = []
         # Figure out what directories are missing.

From efdedaa7b8bab621706f5ea4923d1b6c062d5a32 Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Fri, 30 Jan 2026 11:13:24 +0000
Subject: [PATCH 26/29] Revert tests

---
 Lib/importlib/_bootstrap_external.py | 2 --
 1 file changed, 2 deletions(-)

diff --git a/Lib/importlib/_bootstrap_external.py b/Lib/importlib/_bootstrap_external.py
index 75490acd857d1e..b576ceb1ce9f6e 100644
--- a/Lib/importlib/_bootstrap_external.py
+++ b/Lib/importlib/_bootstrap_external.py
@@ -947,8 +947,6 @@ def _cache_bytecode(self, source_path, bytecode_path, data):
 
     def set_data(self, path, data, *, _mode=0o666):
         """Write bytes data to a file."""
-        # DO NOT MERGE - just testing unconditionally skipping this path
-        return
         parent, filename = _path_split(path)
         path_parts = []
         # Figure out what directories are missing.

From c277edbcc308b80d63d13e16962ad68bb2d745ed Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Fri, 30 Jan 2026 11:31:45 +0000
Subject: [PATCH 27/29] Disable bytecode writing

---
 Python/initconfig.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Python/initconfig.c b/Python/initconfig.c
index 9cdc10c4e78071..a9977738a0c46b 100644
--- a/Python/initconfig.c
+++ b/Python/initconfig.c
@@ -475,7 +475,7 @@ int Py_NoSiteFlag = 0; /* Suppress 'import site' */
 int Py_BytesWarningFlag = 0; /* Warn on str(bytes) and str(buffer) */
 int Py_FrozenFlag = 0; /* Needed by getpath.c */
 int Py_IgnoreEnvironmentFlag = 0; /* e.g. PYTHONPATH, PYTHONHOME */
-int Py_DontWriteBytecodeFlag = 0; /* Suppress writing bytecode files (*.pyc) */
+int Py_DontWriteBytecodeFlag = 1; /* Suppress writing bytecode files (*.pyc) */
 int Py_NoUserSiteDirectory = 0; /* for -s and site.py */
 int Py_UnbufferedStdioFlag = 0; /* Unbuffered binary std{in,out,err} */
 int Py_HashRandomizationFlag = 0; /* for -R and PYTHONHASHSEED */
@@ -1080,7 +1080,7 @@ config_init_defaults(PyConfig *config)
     config->interactive = 0;
     config->optimization_level = 0;
     config->parser_debug= 0;
-    config->write_bytecode = 1;
+    config->write_bytecode = 0;
     config->verbose = 0;
     config->quiet = 0;
     config->user_site_directory = 1;

From 16a75d8fade207e9bbde70170b5c15d6384fbf8d Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Fri, 30 Jan 2026 11:53:45 +0000
Subject: [PATCH 28/29] Revert initconfig.c

---
 Python/initconfig.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Python/initconfig.c b/Python/initconfig.c
index a9977738a0c46b..9cdc10c4e78071 100644
--- a/Python/initconfig.c
+++ b/Python/initconfig.c
@@ -475,7 +475,7 @@ int Py_NoSiteFlag = 0; /* Suppress 'import site' */
 int Py_BytesWarningFlag = 0; /* Warn on str(bytes) and str(buffer) */
 int Py_FrozenFlag = 0; /* Needed by getpath.c */
 int Py_IgnoreEnvironmentFlag = 0; /* e.g. PYTHONPATH, PYTHONHOME */
-int Py_DontWriteBytecodeFlag = 1; /* Suppress writing bytecode files (*.pyc) */
+int Py_DontWriteBytecodeFlag = 0; /* Suppress writing bytecode files (*.pyc) */
 int Py_NoUserSiteDirectory = 0; /* for -s and site.py */
 int Py_UnbufferedStdioFlag = 0; /* Unbuffered binary std{in,out,err} */
 int Py_HashRandomizationFlag = 0; /* for -R and PYTHONHASHSEED */
@@ -1080,7 +1080,7 @@ config_init_defaults(PyConfig *config)
     config->interactive = 0;
     config->optimization_level = 0;
     config->parser_debug= 0;
-    config->write_bytecode = 0;
+    config->write_bytecode = 1;
     config->verbose = 0;
     config->quiet = 0;
     config->user_site_directory = 1;

From c3779af8fc00722ce0043ddb5f36ab4f3d8961eb Mon Sep 17 00:00:00 2001
From: Steve Dower <steve.dower@python.org>
Date: Fri, 30 Jan 2026 15:02:14 +0000
Subject: [PATCH 29/29] Remove do not merge code

---
 Python/sysmodule.c | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 5c819b8d6e0075..a374589145109b 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -1,6 +1,3 @@
-// DO NOT MERGE
-//#define TAG NULL
-
 
 /* System module */
 
@@ -3604,13 +3601,6 @@ make_impl_info(PyObject *version_info)
     if (res < 0)
         goto error;
 
-// DO NOT MERGE
-#ifdef MS_WINDOWS
-    if (GetEnvironmentVariableW(L"PYNOCACHETAG", NULL, 0)) {
-        _PySys_ImplCacheTag = NULL;
-    }
-#endif
-
     value = _PySys_ImplCacheTag
         ? PyUnicode_FromString(_PySys_ImplCacheTag)
         : Py_NewRef(Py_None);