(🐞) Inconsistent reachability depending if module level or not #12409

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1. KotlinIsland

   added

   bugmypy got something wrongmypy got something wrong

   on Mar 21, 2022
2. JelleZijlstra

   added

   topic-reachabilityDetecting unreachable codeDetecting unreachable code

   on Mar 21, 2022
3. ikonst commented on Jun 12, 2023

   

   ikonst

   on Jun 12, 2023

   Last edited by ikonst

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   We give special treatment to top-level assert statements here. This was discussed in #5308 and implemented in #5894.

   The term "unreachable" is overloaded and refers to either:

   • code that cannot be soundly reached

     if False:
       a = 42  # E: Statement is unreachable
     if True:
       raise Exception
       a = 42  # E: Statement is unreachable
     assert False
     a = 42  # E: Statement is unreachable

   • code unreachable on current version/platform

     if sys.version_info < (3,):
        a = 42
     if sys.platform == 'foobar':
        a = 42
     assert sys.platform == 'foobar'
     a = 42

   Maybe it'd be good to refer to the latter as something different than "reachability"

   So, except for this hack, which is intended to reduce the indent in this case

    import sys
   
   -if sys.platform == 'windows':
   -  <rest of file>
   +assert sys.platform == 'windows'
   +<rest of file>

   we don't treat platform/version checks as reachability cues at all (otherwise you'd have to play error whack-a-mole, since if your code has an if-else for two platforms, one would always be unreachable).

   💡 Here's an idea: The #5894 feature is intended to designate an entire module as unfit on other platforms. One clearly has to import sys before this assertion, but it's probably not needed to execute any other statements except for imports, so how about:

   • Continue working as-is if it appears after import statements only.
   • If other statements were seen before it, then it'll be an error, e.g.

   import sys
   
   def f() -> None: ...
   
   assert sys.platform == 'windows'  # E: Module-level platform assertions must appear at the beginning of the module (before other statements)

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