Assertion failure at Python/instruction_sequence.c:121: `int _PyInstructionSequence_Addop(instr_sequence *, int, int, location): Assertion 'OPCODE_HAS_ARG(opcode) || HAS_TARGET(opcode) || oparg == 0' failed`

[YuanchengJiang]

Crash report

What happened?

import unittest
from test.support.bytecode_helper import CodegenTestCase
class IsolatedCodeGenTests(CodegenTestCase):
    def assertInstructionsMatch_recursive(self, insts, expected_insts):
        self.assertInstructionsMatch(insts, expected_insts)
    def codegen_test(self, snippet, expected_insts):
        import ast
        a = ast.parse(snippet, 'my_file.py', 'exec')
        insts = self.generate_code(a)
        self.assertInstructionsMatch_recursive(insts, expected_insts)
    def test_if_expression(self):
        snippet = '42 if True else 24'
        expected = [('RESUME', 0, 0), ('ANNOTATIONS_PLACEHOLDER', None), ('LOAD_CONST', 0, 1), ('TO_BOOL', 256, 1), ('POP_JUMP_IF_FALSE', (false_lbl := 1), 1), ('LOAD_CONST', 1, 1), ('\x00', (exit_lbl := self.Label())), false_lbl, ('', 255, 1), exit_lbl, ('POP_TOP', None), ('LOAD_CONST', 1), ('RETURN_VALUE', None)]
        self.codegen_test(snippet, expected)

if __name__ == "__main__":
    unittest.main(verbosity=0)

python: ../Python/instruction_sequence.c:121: int _PyInstructionSequence_Addop(instr_sequence *, int, int, location): Assertion `OPCODE_HAS_ARG(opcode) || HAS_TARGET(opcode) || oparg == 0' failed.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

No response

[johnslavik]

Minimal reproducer:

import _testinternalcapi
seq = _testinternalcapi.new_instruction_sequence()
seq.addop(39, 256, 1, -1, -1, -1)

[johnslavik]

Your test calls addop improperly.

If you fix your test to pass 0 and not 256 as the oparg:

<             ("TO_BOOL", 256, 1),
>             ("TO_BOOL", 0, 1),

it will no longer trigger the C-level assertion error.

That said, the underlying incorrect call is:

seq.addop(39, 256, 1, -1, -1, -1)

It should be:

seq.addop(39, 0, 1, -1, -1, -1)

TO_BOOL (opcode 39) does not take an argument, but the test does the addop call with 256 as the oparg instead of the expected 0.

That's precisely what the assertion says:

cpython/Python/instruction_sequence.c

Line 121 in 3e36d37

assert(OPCODE_HAS_ARG(opcode) || HAS_TARGET(opcode) || oparg == 0);

Any op has to either:

• take an argument (TO_BOOL doesn't) or
• be a jumping instruction (such as in for) or a "block-push" instruction (such as in with) (TO_BOOL doesn't) or
• receive 0 as the argument, which is valid for opcodes that don't take arguments (TO_BOOL expected 0, but 256 was passed).

@iritkatriel, do you think that InstructionSequenceType_addop_impl should throw exceptions for incorrect calls before reaching _PyInstructionSequence_Addop? Or is this just invalid, and the caller is fully responsible for abiding the contract?