Assert statements in Enum subclass definitions case errors when assertion rewriting is enabled

[finite-state-machine]

While running pytest with the following code:

from enum import Enum

STEP = 100

class SomeEnum(Enum):

    FIRST = 100
    SECOND = FIRST + STEP
    THIRD = SECOND + STEP

    assert THIRD == 300

...assertion rewriting causes an error because of an interaction with Enum:

================================ test session starts ================================
platform darwin -- Python 3.8.12, pytest-6.2.5, py-1.9.0, pluggy-0.13.1
rootdir: /private/tmp/pytest-enum-assert-bug
plugins: profiling-1.7.0, order-1.0.1, cov-3.0.0
collected 0 items / 1 error

====================================== ERRORS =======================================
______________________________ ERROR collecting bug.py ______________________________
bug.py:5: in <module>
    class SomeEnum(Enum):
bug.py:11: in SomeEnum
    assert THIRD == 300
/Users/dsuffling/.pyenv/versions/3.8.12/lib/python3.8/enum.py:112: in __setitem__
    raise TypeError('Attempted to reuse key: %r' % key)
E   TypeError: Attempted to reuse key: '@py_assert1'
============================== short test summary info ==============================
ERROR bug.py - TypeError: Attempted to reuse key: '@py_assert1'
!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!
================================= 1 error in 0.28s ==================================

This error does not occur with --assert=plain:

================================ test session starts ================================
platform darwin -- Python 3.8.12, pytest-6.2.5, py-1.9.0, pluggy-0.13.1
rootdir: /private/tmp/pytest-enum-assert-bug
plugins: profiling-1.7.0, order-1.0.1, cov-3.0.0
collected 0 items

=============================== no tests ran in 0.02s ===============================

In fact, the issue can be reproduced by:

from enum import Enum

class SomeEnum(Enum):
    assert True

Environment:

pytest version: 6.2.5
environment: macOS 10.14, homebrew 3.3.12, pyenv 2.2.3, CPython 3.8.12
pip list (new virtualenv; nothing but pytest installed):

Package    Version
---------- -------
attrs      21.4.0
iniconfig  1.1.1
packaging  21.3
pip        21.1.1
pluggy     1.0.0
py         1.11.0
pyparsing  3.0.7
pytest     6.2.5
setuptools 56.0.0
toml       0.10.2

[RonnyPfannschmidt]

@finite-state-machine adding the magic value PYTEST_DONT_REWRITE to the docstring of the module can help as a temporary workaround

[RonnyPfannschmidt]

@nicoddemus i believe we may have to opt out of assertion rewriting in class bodies and/or turn the assert statement into a traceback hiding function

this may also affect other classes that use own namespace types

[nicoddemus]

i believe we may have to opt out of assertion rewriting in class bodies

@RonnyPfannschmidt why do you say that? At first glance it seems it should be possible to fix the problem (but I have not looked into it).

[RonnyPfannschmidt]

@nicoddemus the execution namespace in a class may have special meaning, so we ought not to affect it in such cases

[yuvalshi0]

Just from quick analysis of this code,
The issue come from rewrite [temporary variables clear].(https://github.com/yuvalshi0/pytest/blob/8b211983ff11be5238f90bfd0b3523f03fea09ae/_pytest/assertion/rewrite.py#L463)

From reading this part I assume what pytest actually runs is:

class SomeEnum(Enum):
    @py_assert0 = True
    ...  # rewrite magic
    @py_assert = None

Causing a double Enum key statement inside the Enum class body.
I personally think there is no a straightforward fix for this issue, even if you detect the assert statement is inside a class body, you don't have any way to know that it does not allow duplicate members, such as this case.