re.Match.group() output doesn't raise type error

[john-dicarlo]

# example.py

import re

def foo(s: str) -> str:
   print(s)
   return s

m: re.Match[str] | None = re.match('(a)(b)?', 'a')

if m is not None:
   foo(None) # Argument 1 to "foo" has incompatible type "None"; expected "str"  [arg-type] # as expected
   
   reveal_type(m.group(2))  # Revealed type is "Union[builtins.str, Any]"
   foo(m.group(2)) # does not raise an [arg-type] error
   assert m.group(2) is None # passes

$ mypy --strict example.py 
example.py:12: error: Argument 1 to "foo" has incompatible type "None"; expected "str"  [arg-type]
example.py:14: note: Revealed type is "Union[builtins.str, Any]"
Found 1 error in 1 file (checked 1 source file)

Why doesn't m.group(2) raise an [arg-type] error here?

Your Environment

mypy 1.6.1 (compiled: yes)
Python 3.10.12

[hauntsaninja]

See python/typeshed#9465 (comment) / python/typeshed#9482 (comment)

There's a balance between false negatives (like this) and false positives. Typeshed aims to avoid false positives, which is why the return type uses a union containing Any. For many — maybe even most — regular expressions, group can't actually be None, so this is a good decision.

[KotlinIsland]

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)