multiprocessing: Value / Array with lock=False are not Sized

[prauscher]

Hello everyone,

Consider the following code:

from multiprocessing import Array
from ctypes import c_char

a = Array(c_char, 128, lock=True)
print(len(a))  # this is fine

b = Array(c_char, 128, lock=False)
print(len(b))  # this is: Argument 1 to "len" has incompatible type "c_char"; expected "Sized"

To my untrained eye, the issue is with https://github.com/python/typeshed/blob/main/stdlib/multiprocessing/sharedctypes.pyi#L40 returning _CT while https://github.com/python/typeshed/blob/main/stdlib/multiprocessing/sharedctypes.pyi#L48 returns SynchronizedArray[Any]

Actually, Array does support __len__ even with lock=False and therefore is Sized. I'm quite sure there is a reason to return _CT instead for lock=False, but I do not know it and it probably should not have this side effect.

[srittau]

I have no experience with sharedctypes, but this looks to be to be a simple bug: b should return _SomeArrayType[_CT], instead of c_char. At runtime, the type returned identifies as multiprocessing.sharedctypes.c_char_Array_128, but that type doesn't exist. I would think we need to defined a type-specific array type and use that. But maybe someone with more sharedctypes experience can chime in.

[Akuli]

I think it should be ctypes.Array[_CT]. It would be consistent with how the * operator is annotated, and the * operator is how you usually create an array type in ctypes:

>>> import ctypes
>>> ctypes.c_char * 128
<class '__main__.c_char_Array_128'>

[ankm20]

Can this be assigned to me?

[prauscher]

Thanks @Akuli, i tested your suggestion by replacing _CT with ctypes.Array[_CT] in

typeshed/stdlib/multiprocessing/context.pyi

Line 90 in 3802899

def Array(self, typecode_or_type: type[_CT], size_or_initializer: int | Sequence[Any], *, lock: Literal[False]) -> _CT: ...

So while this would solve the issue, I'm not sure about possible side effects...

[srittau]

You can always open an exploratory PR here. mypy_primer will test the change against quite a lot of projects, which helps us to judge the impact.

[prauscher]

I created #11576 to test this change, maybe someone can help with review there?