mypy currently infers the type of attrs.fields(foo) as Any, where foo is an instance of an attrs class. Is it possible to have mypy infer the correct type?
With this setup:
import attrs
from attrs import define
@define
class Foo:
bar: int
foo = Foo(1)
mypy infers the correct type for attrs.fields(Foo):
fields = attrs.fields(Foo)
reveal_type(fields)
reveal_type(fields.bar)
reveal_type(fields.not_bar)
"""
demo.py:13: note: Revealed type is "tuple[attr.Attribute[builtins.int], fallback=demo.Foo.__demo_Foo_AttrsAttributes__]"
demo.py:14: note: Revealed type is "attr.Attribute[builtins.int]"
demo.py:15: error: "__demo_Foo_AttrsAttributes__" has no attribute "not_bar" [attr-defined]
demo.py:15: note: Revealed type is "Any"
Found 1 error in 1 file (checked 1 source file)
"""
but mypy infers attrs.fields(type(foo)) as Any:
fields = attrs.fields(type(foo))
reveal_type(fields)
reveal_type(fields.bar)
reveal_type(fields.not_bar)
"""
demo.py:13: note: Revealed type is "Any"
demo.py:14: note: Revealed type is "Any"
demo.py:15: note: Revealed type is "Any"
Success: no issues found in 1 source file
"""
Pitch
Passing the class to attrs.fields() is the documented usage.
If you pass type(attrs_instance) instead, mypy quietly treats the type as Any (even under strict mode), so it cannot catch attr-defined errors when you access a non-existent field.
Feature
mypy currently infers the type of
attrs.fields(foo)
asAny
, wherefoo
is an instance of anattrs
class. Is it possible to have mypy infer the correct type?With this setup:
mypy infers the correct type for
attrs.fields(Foo)
:but mypy infers
attrs.fields(type(foo))
asAny
:Pitch
Passing the class to
attrs.fields()
is the documented usage.If you pass
type(attrs_instance)
instead, mypy quietly treats the type asAny
(even understrict
mode), so it cannot catchattr-defined
errors when you access a non-existent field.ref https://github.com/python-attrs/attrs/issues/1297