Don't issue `no-redef` with conditional imports

[bersbersbers]

Bug Report

When an exception is caught from a block with a single import statement, one can be certain that the import failed (right?).

When an import fails, a to-be-imported function is not defined.

If a function is not defined, mypy should not issue a no-redef message.

To Reproduce

pip install overrides

"""Feature request: detect failed imports and suppress no-redef."""
try:
    from overrides import overrides
except ModuleNotFoundError:
    # mypy issues no-redef
    def overrides() -> None:
        pass

print(overrides)

Expected Behavior

No message

Actual Behavior

bug.py:5: error: Name "overrides" already defined (possibly by an import) [no-redef]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 0.982 (compiled: yes)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10.8

[bersbersbers]

Interestingly, the same is not happening in this example:

"""Counter-example (probably due to matplotlib not being typed)"""
try:
    from matplotlib.cm import jet
except ModuleNotFoundError:
    def jet() -> None:  # mypy does not issue no-redef
        pass

print(jet)

(Which is because the import statement is incorrect.)

[bersbersbers]

A workaround can be found in mkorpela/overrides#109 (comment).

[bersbersbers]

@hauntsaninja I may be missing something, but the code example I posted in #13914 (comment) still issues the same mypy error:

C:\Code\project>mypy --version
mypy 1.0.0+dev.a4da89e0543ee0d213b30216c9e15a5d00fcd578 (compiled: no)

C:\Code\project>mypy --no-incremental project\project.py
project\project.py:6: error: Name "overrides" already defined (possibly by an import)  [no-redef]
Found 1 error in 1 file (checked 1 source file)

Using overrides==7.3.1

[bersbersbers]

Just for the record (because I had been hoping it might have been fixed), this is still an issue on 1.0.0.

[bersbersbers]

Some more details:

"""With `overrides==7.3.1` installed, mypy issues `no-redef` in `except`."""
from typing import Callable, TypeVar

F = TypeVar("F", bound=Callable)
try:
    from overrides import override
except ModuleNotFoundError:
    def override(method: F) -> F:
        return method

print(override)

The workaround works , but stops working when removing overrides:

"""The workaround silences `mypy`, but now mypy issues `no-redef` in *`try`* if `overrides` is *not* installed."""
from typing import Callable, TypeVar

F = TypeVar("F", bound=Callable)

# Workaround
override: Callable[[F], F]
try:
    from overrides import override
except ModuleNotFoundError:
    def override(method: F) -> F:
        return method

print(override)

This is a problem if one uses a construct such as this to import typing.override (from Python 3.12 on):

"""Prepare for Python 3.12."""
from typing import Callable, TypeVar

F = TypeVar("F", bound=Callable)

# Workaround
override: Callable[[F], F]
try:
    from typing import override
except ModuleNotFoundError:
    def override(method: F) -> F:
        return method

print(override)

For the record, this works:

"""Prepare for Python 3.12."""
import sys
from typing import Callable, TypeVar

F = TypeVar("F", bound=Callable)

if sys.version_info >= (3, 12):
    from typing import override
else:
    def override(method: F) -> F:
        return method

print(override)