mypy does not recognize sys.platform with ternary operator

[amatyukhin0]

Bug Report

mypy can recognize sys.platform in if cond: ... / else: ... conditions but fails to do it if ternary operator (a if cond else b) is used.

To Reproduce

Consider this piece of code, assuming we are on Linux:

import signal
import sys

signal_type = signal.SIGBREAK if sys.platform == 'win32' else signal.SIGTERM

Expected Behavior

mypy should not raise any error.

Actual Behavior

mypy fails with:

main.py:4: error: Module has no attribute "SIGBREAK"
Found 1 error in 1 file (checked 1 source file)

Environment

• Mypy version used: 0.961
• Python version used: 3.10
• Operating system: Linux

[AlexWaygood]

I think this is better classified as a feature rather than a bug, since PEP 484 states:

Type checkers are expected to understand simple version and platform checks

(Emphasis mine on "simple".)

[jaraco]

I'd argue a ternary expression is more simple than if/else blocks:

• A ternary expression has exactly one assignment, so there's less ambiguity around the effect of the conditional. One variable gets assigned unconditionally or not at all.
• It's not possible to interleave other concerns, such as assigning other variables.
• It's not possible to have arbitrary side-effects in either logical branch (e.g. sys.path.insert()).

These are the same reasons that I encourage use of the ternary expression, as these constraints also help protect the logical safety of the operation.

</soapbox>