Makes Array abstract

[sobolevn]

Now, this code:

from ctypes import Array, c_int

class I(Array[c_int]):
    ...

Array()
I()

Raises:

out/ex.py:6: error: Cannot instantiate abstract class "Array" with abstract attributes "_length_" and "_type_"
out/ex.py:7: error: Cannot instantiate abstract class "I" with abstract attributes "_length_" and "_type_"

Tested locally with recent mypy.

Refs #6349

[sobolevn]

I have to support _type_: str as well. Like here https://github.com/python/cpython/blob/f4c03484da59049eb62a9bf7777b963e2267d187/Modules/_ctypes/_ctypes.c#L1020

[sobolevn]

I have to support _type_: str as well. Like here

Or not. It does not work in runtime. There are just some confusing examples in the CPython's source code.

[AlexWaygood]

This feels like it maybe relies on a mypy bug at the moment. Adding abstractmethod to a class's methods shouldn't make the class abstract unless the class uses an ABCMeta-derived as its metaclass, and ctypes.Array doesn't do that in the stubs at the moment. The fact that mypy nonetheless recognises ctypes.Array as abstract, even though it doesn't have an ABCMeta-derived class as its metaclass, feels like something of a bug in mypy 🙂

[JukkaL]

The fact that mypy nonetheless recognises ctypes.Array as abstract, even though it doesn't have an ABCMeta-derived class as its metaclass, feels like something of a bug in mypy

This is by design. It allows defining ABCs that are only checked during static type checking but not at runtime. This can be helpful to avoid metaclass conflicts, for example.

[AlexWaygood]

This is by design. It allows defining ABCs that are only checked during static type checking but not at runtime. This can be helpful to avoid metaclass conflicts, for example.

Thanks, TIL!

[sobolevn]

Any ideas how can we fix this pytype problem? https://github.com/python/typeshed/runs/4297765776?check_suite_focus=true (Sorry, I've never used pytype)

stdlib/ctypes/__init__.pyi (3.9): ParseError: Overloaded signatures for _length_ disagree on abstractmethod decorators

First idea

This does not work with mypy:

    @property
    @abstractmethod
    def _length_(self) -> int: ...
    @_length_.setter
    @abstractmethod
    def _length_(self, value: int) -> None: ...

    @property
    @abstractmethod
    def _type_(self) -> type: ...
    @_type_.setter
    @abstractmethod
    def _type_(self, value: type) -> None: ...

out/ex.py:4: error: Overloaded method has both abstract and non-abstract variants
out/ex.py:7: error: Decorated property not supported
out/ex.py:11: error: Overloaded method has both abstract and non-abstract variants
out/ex.py:14: error: Decorated property not supported

This does not seem right to me: looks like this code should be accepted by mypy:

1. It works fine in runtime
2. It is documented here: https://docs.python.org/3/library/abc.html#abc.abstractproperty

I will open a new issue for it: python/mypy#11601

[Akuli]

CC @rchen152

[rchen152]

Huh, looks like the issue might be that pytype thinks that the property and setter methods are overloads? I'll take a look.

[rchen152]

I've sent a fix for the pytype issue out for review. Hopefully, I'll be able to do a release tomorrow; if not, it'll have to wait until next Monday (after Thanksgiving).

[rchen152]

I've released pytype-2021.11.24, which should fix the pytype_test failure.

[sobolevn]

@rchen152 thank you! 🎉

[Akuli]

ctypes.Array._length_ and ctypes.Array._type_ must be added to tests/stubtest_allowlists/py3_common.txt.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[sobolevn]

Thanks everyone!