Assertion failure in Python/generated_cases.c.h `PyObject *_PyEval_EvalFrameDefault(PyThreadState *, _PyInterpreterFrame *, int): Assertion 'oparg == co->co_nfreevars' failed`

[YuanchengJiang]

Crash report

What happened?

import opcode
from types import FunctionType

COPY_FREE_VARS = opcode.opmap.get('COPY_FREE_VARS', None)
if COPY_FREE_VARS is None:
    raise SystemExit("skip: opcode COPY_FREE_VARS not available on this Python")

def external_getitem(self, i):
    return f'Foreign getitem: {super().__getitem__(i)}'

def create_closure(__class__):
    return (lambda: __class__).__closure__

def new_code(c):
    prep = bytes([COPY_FREE_VARS, 0])
    return c.replace(
        co_freevars=c.co_freevars + ('__class__',),
        co_code=prep + c.co_code
    )

def add_foreign_method(cls, name, f):
    code = new_code(f.__code__)
    closure = create_closure(cls)
    defaults = f.__defaults__
    setattr(cls, name, FunctionType(code, globals(), name, defaults, closure))

class List(list):
    pass

add_foreign_method(List, '__getitem__', external_getitem)
obj = List([1, 2, 3])
result = obj[0]

python: ../Python/generated_cases.c.h:5129: PyObject *_PyEval_EvalFrameDefault(PyThreadState *, _PyInterpreterFrame *, int): Assertion `oparg == co->co_nfreevars' failed.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

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

No response

[picnixz]

We won't fix this as per #140776 (comment) because crashers arising from messing with code objects cannot be fixed easily. Please do not post such crashers in the future unless you are sure that the replacement is legitimate.