gh-142651: Make call_count of Mock thread safe

Lib/test/test_unittest/testmock/testthreadingmock.py

@@ -1,5 +1,6 @@
 import time
 import unittest
+import threading
 import concurrent.futures
 
 from test.support import threading_helper
@@ -196,6 +197,27 @@ def test_reset_mock_resets_wait(self):
         m.wait_until_any_call_with()
         m.assert_called_once()
 
+    def test_call_count_thread_safe(self):
+
+        m = ThreadingMock()
+
+        # 3k loops reliably reproduces the issue while keeping runtime ~0.6s
+        LOOPS = 3_000
+        THREADS = 10
+
+        def test_function():
+            for _ in range(LOOPS):
+                m()
+
+        threads = [threading.Thread(target=test_function) for _ in range(THREADS)]
+        for thread in threads:
+            thread.start()
+        for thread in threads:
+            thread.join()
+
+        self.assertEqual(m.call_count, LOOPS * THREADS,
+                        f"Expected {LOOPS * THREADS}, got {m.call_count}")
+
 
 if __name__ == "__main__":
     unittest.main()

Lib/unittest/mock.py

@@ -1180,14 +1180,14 @@ def _mock_call(self, /, *args, **kwargs):
 
     def _increment_mock_call(self, /, *args, **kwargs):
         self.called = True
-        self.call_count += 1
 
         # handle call_args
         # needs to be set here so assertions on call arguments pass before
         # execution in the case of awaited calls
         _call = _Call((args, kwargs), two=True)
         self.call_args = _call
         self.call_args_list.append(_call)
+        self.call_count = len(self.call_args_list)
 
         # initial stuff for method_calls:
         do_method_calls = self._mock_parent is not None

Misc/NEWS.d/next/Library/2025-12-13-06-17-44.gh-issue-142651.ZRtBu4.rst

@@ -0,0 +1 @@
+Fixed a thread safety issue in :mod:`unittest.mock` where :attr:`~unittest.mock.Mock.call_count` could return inaccurate values when a mock was called concurrently from multiple threads. The attribute now derives its value from the length of :attr:`~unittest.mock.Mock.call_args_list` to ensure consistency.