E0601: used-before-assignment false positive for import under TYPE_CHECKING check

[Rogdham]

Bug description

# pylint: disable=missing-module-docstring, missing-function-docstring

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from typing_extensions import assert_type
else:

    def assert_type(val, _):
        return val

ANSWER = 42
assert_type(ANSWER, int)

Configuration

No response

Command used

pylint a.py

Pylint output

************* Module a
a.py:13:0: E0601: Using variable 'assert_type' before assignment (used-before-assignment)

------------------------------------------------------------------
Your code has been rated at 2.86/10 (previous run: 2.86/10, +0.00)

Expected behavior

This is a false positive, as assert_type is defined in both branches of the if.

Pylint version

pylint 2.15.0
astroid 2.12.4
Python 3.10.6 (main, Aug  3 2022, 17:39:45) [GCC 12.1.1 20220730]

OS / Environment

Archlinux

Additional dependencies

astroid==2.12.4
dill==0.3.5.1
isort==5.10.1
lazy-object-proxy==1.7.1
mccabe==0.7.0
platformdirs==2.5.2
pylint==2.15.0
tomli==2.0.1
tomlkit==0.11.4
wrapt==1.14.1

[Rogdham]

The issue is still here if the import is from stdlib instead of an external module:

# pylint: disable=missing-module-docstring, missing-function-docstring

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from collections import defaultdict
else:
    def defaultdict():
        return {}

defaultdict()

************* Module a
c.py:11:6: E0601: Using variable 'defaultdict' before assignment (used-before-assignment)

------------------------------------------------------------------
Your code has been rated at 1.67/10 (previous run: 1.67/10, +0.00)

[jacobtylerwalls]

Good catch. 3159b17 only checked AssignName nodes, not FunctionDef nodes, as here. Would you like to prepare a PR?

[DanielNoord]

@jacobtylerwalls Just for my understanding, why are we special-casing TYPE_CHECKING here?

# pylint: disable=missing-module-docstring, missing-function-docstring

from typing import value

if value:
    from typing_extensions import assert_type
else:

    def assert_type(val, _):
        return val


ANSWER = 42
assert_type(ANSWER, int)

This doesn't raise, only a no-name-in-module warning.

[jacobtylerwalls]

Assignments under if TYPE_CHECKING are always false at runtime, so from the perspective of used-before-assignment, we can assume they never happened. With some control-flow knowledge that the name is defined in the else, though, we can avoid assuming the name was never assigned. Here the else is only checked for assignments, not function definitions.

[Rogdham]

Would you like to prepare a PR?

@jacobtylerwalls I did what I could in #7370, but the pylint codebase is foreign to me so I'm sure I missed tons of things!