Pylance reports code is unreachable in if/else blocking checking sys.platform

[boydgreenfield]

Environment data

• Language Server version: 2024.8.1
• OS and version: darwin arm64
• Python version (and distribution if applicable, e.g. Anaconda):
• python.analysis.indexing: true
• python.analysis.typeCheckingMode: off

Code Snippet

patch_args = ["patch", "--fuzz", "0", "--unified"]
if sys.platform == "darwin":
    patch_args.append("--check")
else:
    patch_args.append("--dry-run")

This code works but feels, well, obtuse:

patch_args = ["patch", "--fuzz", "0", "--unified"]
platform: str = sys.platform
if platform == "darwin":
    patch_args.append("--check")
else:
    patch_args.append("--dry-run")

Expected behavior

No problem raised on if sys.platform ... block

Actual behavior

Problem is raised.

[debonte]

This is by design. There's some discussion about this in these pyright issues:

• Type checker reports code inside sys.platform check as unreachable pyright#410
• "assert sys.platform == 'linux'" incorrectly marks code unreachable pyright#621

The key part IMO is this:

Pyright needs to make assumptions about the value of sys.platform during analysis because many type stub files (including the typeshed stub files for stdlib) use sys.platform to conditionalize code for different platforms. By default, Pyright assumes that sys.platform is "" (empty string). This is an appropriate assumption for code that is intended to run on all platforms. If you intend to run your code on a specific platform, you can specify the target in the pythonconfig.json file using the pythonPlatform setting. If you set this to "Linux", Pyright will assume that sys.platform == 'linux'.