Callable[[Any], None]" has no attribute "__defaults__"

[jpy-git]

Bug Report

mypy produces the false positive Callable[[Any], None]" has no attribute "__defaults__" when code uses the __defaults__ attribute of a function.

To Reproduce

Consider this code:

>>> def foo(a) -> None: pass
>>> print(foo.__defaults__)  # None if no default arguments.
None
>>> def bar(a="abc") -> None: pass
>>> print(bar.__defaults__)  # tuple of default arguments.
("abc",)

Run mypy on a script containing this code and get the false-positive:

test.py:2: error: "Callable[[Any], None]" has no attribute "__defaults__"
test.py:4: error: "Callable[[Any], None]" has no attribute "__defaults__"
Found 2 errors in 1 file (checked 1 source file)

Expected Behavior

No mypy errors since it's a valid attribute of a function.

Actual Behavior

mypy errors shown above.

Your Environment

• Mypy version used: mypy 0.930
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: Python 3.9.9
• Operating system and version: MacOS Big Sur

[AlexWaygood]

It looks to me like the issue is that the definition for the (fictional) function class in typeshed/stdlib/builtins.pyi has not got many of the attributes that the (non-fictional) FunctionType class in typeshed/stdlib/types.pyi has.

You can get similar mypy errors for the following attributes, which are also defined on types.FunctionType, but not on builtins.function:

• foo.__closure__
• foo.__kwdefaults__
• foo.__get__

[jpy-git]

@AlexWaygood Is there any reason we can't just have builtins.function be a type alias for types.FunctionType since they're the same?
i.e. function = types.FunctionType

If not then I'm happy to just copy the attributes over from FuntionType 😄

[AlexWaygood]

There have been previous attempts to delete builtins.function, which have (sadly) gone badly (see python/typeshed#5799), but I'm not aware of any previous attempts to simply have builtins.function be a type alias for types.FunctionType. Try it, see what happens! 😀

[jpy-git]

Closing as we've fixed this in typeshed 👍