type narrowing leads to "unreachable" code being actually reachable #12598

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1. Akuli

   added

   bugmypy got something wrongmypy got something wrong

   on Apr 16, 2022
2. AlexWaygood

   added

   topic-reachabilityDetecting unreachable codeDetecting unreachable code

   on Apr 16, 2022
3. beauxq commented on Apr 29, 2022

   

   beauxq

   on Apr 29, 2022

   Last edited by beauxq

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   This is a section of my test suite code (not even a made up example):

       assert not vm.paused
       press_key(pygame.K_SPACE)
       assert vm.paused
       press_key(pygame.K_SPACE, True)
       assert not vm.paused

   mypy is reporting unreachable code on the 4th line
   That is wrong. That code is reachable and it is executed when I run it.

   It is assuming that press_key can't change vm.paused, which is a false assumption.
4. beauxq commented on Apr 29, 2022

   

   beauxq

   on Apr 29, 2022

   More actions

   re: the "topic-reachability" tag

   I think this is more of a type-narrowing issue, rather than reachability.

   To narrow types, you have to look at the scope of the name being narrowed (the narrowee), and the scopes of all of the functions called during the narrowed time.
   As soon as a function is called that could possibly access the scope of the narrowee, the narrowing is gone.

   If someone wants to make sure their type gets narrowed, they can make a local variable, and then only call functions that are defined outside of the local scope.
5. Akuli

   changed the title [-]if statements type narrowing leads to "unreachable" code being actually reachable[/-] [+]type narrowing leads to "unreachable" code being actually reachable[/+] on Apr 29, 2022