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re.Match.group() output doesn't raise type error #16441
Description
# 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
Activity
hauntsaninja commentedon Nov 12, 2023
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 commentedon Jan 18, 2024
Basedmypy works correctly in these cases: