ctypes.Array should be abstract

[sobolevn]

Right now Array in ctypes is a regular class:

typeshed/stdlib/ctypes/__init__.pyi

Lines 269 to 271 in 5c8e68f

	class Array(Generic[_CT], _CData):
	_length_: ClassVar[int]

But, in reallity - you cannot instantiate it:

>>> from ctypes import Array, c_int
>>> i = c_int(1)
>>> Array(i)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: abstract class

Mypy is also not happy with Array(...) but for other reasons:

out/ex.py:4: error: Array constructor argument 1 of type "c_int" is not convertible to the array element type <nothing>
out/ex.py:4: note: Revealed type is "ctypes.Array[<nothing>]"

Array(...) is not tested in https://github.com/python/mypy/blob/master/test-data/unit/check-ctypes.test at all

[JelleZijlstra]

Note that mypy has a dedicated plugin for ctypes (https://github.com/python/mypy/blob/master/mypy/plugins/ctypes.py), so not everything here may be fixable in typeshed.

[srittau]

I don't think there is a good way to make Array abstract in typeshed. As it doesn't derive from abc.ABCMeta at runtime, we shouldn't derive it either. If you have an idea on how to mark it, please open a PR. Until then I'm closing this as "inexpressable".

[sobolevn]

We can try to make _length_ and _type_ abstract properties: https://github.com/python/cpython/blob/main/Modules/_ctypes/_ctypes.c#L1522-L1526

Docs: https://github.com/python/cpython/blob/1cbaa505d007e11c4a1f0d2073d72b6c02c7147c/Doc/library/ctypes.rst#arrays-and-pointers

This way, we would have a class with "abtract" attributes and it won't be possible to create instances of it.
It would be also helpful for subclassing like

class X(object, Array):
   _length_ = 5
   _type_ = "i"

[sobolevn]

But, they should be writable:

>>> from ctypes import c_int
>>> x = c_int * 3
>>> x
<class '__main__.c_int_Array_3'>
>>> x._length_
3
>>> x._length_ = 2
>>> x._type_ = 'a'

[sobolevn]

Refs #6348