Reason for re.Match.group and re.Match.__getitem__ returning AnyStr | Any? #9465

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1. AlexWaygood commented on Jan 6, 2023

   

   AlexWaygood

   on Jan 6, 2023

   Last edited by AlexWaygood

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   There's been a lot of discussion of the signature of Match.group() over the years:

   • Wrong return type for Match.group() #3171
   • Fix Match.group problems #3172
   • Partially restore old (imprecise) signature of Match.group() #3190
   • Match.group can return None #3902
   • Rework Match.group handling #5557

   Trying to piece together a precise history at this point is quite honestly a little difficult! I think it's likely that the stricter, more correct signature causes an unacceptable number of false positives. But you're welcome to submit a PR and see what the results of mypy_primer are in 2023 :-) (No promises that it will necessarily be merged.)
2. AlexWaygood commented on Jan 9, 2023

   

   AlexWaygood

   on Jan 9, 2023

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   Well, yup, looks like AnyStr | Any is still the best we can do here for now! #9482 (comment)
3. AlexWaygood

   closed this as not plannedon Jan 9, 2023
4. AlexWaygood commented on Jan 9, 2023

   

   AlexWaygood

   on Jan 9, 2023

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   (Thanks for the PR, by the way! It's always good to check whether these decisions made years ago still apply. Type checkers are always getting better with these things, and experimenting is definitely encouraged in this repo :)
5. willfrey commented on Jan 9, 2023

   

   willfrey

   on Jan 9, 2023

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   Thanks! I get annoyed with Any creeping its way into my code, so I try to stamp that out when I can. Problem is, I still don't have a good intuition of when I should use cast() vs. an explicit type annotation for a variable vs. some sort of assertion to narrow the inferred type, especially as it pertains when something has type Any or some union with Any.

   Taking an example right from the re docs,

   import re
   
   m = re.match(r"(\w+) (\w+)", "Isaac Newton, physicist")
   assert m is not None
   first_name = m[1]  # Has type `str | Any`
   
   # Do this?
   g1: str = m[1]
   
   # Or this?
   g1 = cast(str, m[1])
   
   # Or this?
   g1 = m[1]
   assert isinstance(g1, str)

   The first two make disallow-any-expr directives happy whereas the third one does not.

   @AlexWaygood Do you have any personal philosophy of when to use what? If so, I'd love to hear it!
6. hauntsaninja commented on Jan 9, 2023

   

   hauntsaninja

   on Jan 9, 2023

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   The first is more type safe, cast will let you lie

   👍React with 👍2Reacted by Alex Waygood and Sebastian Rittau
7. willfrey commented on Jan 9, 2023

   

   willfrey

   on Jan 9, 2023

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   @hauntsaninja Yeah, agreed about cast. It's the BFG of shutting up the type checker.
8. KotlinIsland commented on Jan 18, 2024

   

   KotlinIsland

   on Jan 18, 2024

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   Basedmypy works correctly in these cases:

   import re
   
   s: str
   if m := re.match("a(b)?(c)", s):
       reveal_type(m.groups())  #  (str | None, str)