Consider adding __slots__ to stubs?

Since mypy now supports __slots__ checking, we can add __slots__ to stubs.
This will allow us to correctly raise Something is not in __slots__ when assigning extra attributes.

Right now there are no __slots__ defined: https://github.com/python/typeshed/search?q=__slots__

If this is something we are going for, there are several things to consider:

Adding this feature to stubtest. It can check that __slots__ contents do match. Probably behind a feature flag. We can enable it for stdlib only
Should we allow skipping _protected attributes? For example, if some class is defined as
```
class A:
   __slots__ = ('_prop',)
```
Should we allow this stub?
```
class A:
    __slots__ = ()
```

Adding this feature to stubtest. It can check that __slots__ contents do match. Probably behind a feature flag. We can enable it for stdlib only

It's currently included in stubtest's list of always-ignored dunders inside classes: https://github.com/python/mypy/blob/dc5c299aa190949f2300b163ccc10257a779006d/mypy/stubtest.py#L1280-L1281.

Technically it's not always ignored, it's "ignorable". Aka if the runtime has __slots__, stubtest won't currently enforce that the stub has __slots__. But if the stub does have __slots__, stubtest will make sure they match the runtime.

I agree that adding __slots__ is useful in stubs.

For your question 2, I think it's best to add protected attributes. That way, we can just have the slots definition match exactly between runtime and the stub, which is easier to test and review.

Ok, I will start with stubtest PR! 👍

to correctly raise Something is not in slots when assigning extra attributes.

Can you clarify this? Type checkers complain about assigning non-existent attributes anyway, unless it's a subclass and then the __slots__ of the parent class doesn't matter anyway at runtime (unless the subclass defines its own __slots__, in that case the fields that are actually accessible are Parent.__slots__ + Child.__slots__).

@Akuli

As @Akuli says, I don't think adding __slots__ to the stub helps much with whether or not type checkers will validate attribute assignments.

Where it might help is in code like this:

>>> class A:
...     __slots__ = ["foo"]
... 
...     
>>> class B:
...     __slots__ = ("bar",)
... 
...     
>>> A.__slots__ + B.__slots__
Traceback (most recent call last):
  File "<pyshell#6>", line 1, in <module>
    A.__slots__ + B.__slots__
TypeError: can only concatenate list (not "tuple") to list
>>> class C(A, B): ...
... 
Traceback (most recent call last):
  File "<pyshell#9>", line 1, in <module>
    class C(A, B): ...
TypeError: multiple bases have instance lay-out conflict

Adding __slots__ to the stub could plausibly help type checkers catch both of those exceptions.

Sorry, an example was needed 🙂

Mypy has a special feature to check attribute definitions to be compatible with __slots__: https://github.com/python/mypy/blob/dc5c299aa190949f2300b163ccc10257a779006d/mypy/checker.py#L3196-L3225
Tests can be found here: https://github.com/python/mypy/blob/master/test-data/unit/check-slots.test

So, let's say we have a definition of SomeStub type in typeshed, which does not have __slots__ defined:

class SomeStub: ...  # defined in typeshed


class A(SomeStub):  # user's code
    __slots__ = ("b",)

    def __init__(self) -> None:
        # All three will pass, because `SomeStub` parent has `__dict__`:
        self.a = 1
        self.b = 2
        self._one = 1

Compare it to the next case, where we have __slots__ defined:

class SomeStubWithSlots:  # defined in typeshed
    __slots__ = ("a",)


class A(SomeStubWithSlots):  # user's code
    __slots__ = ("b",)

    def __init__(self) -> None:
        self.a = 1
        self.b = 2
        self._one = 1  # E: Trying to assign name "_one" that is not in "__slots__" of type "ex.A"

I now see how it's useful. It is an improvement, but also not super important because new attributes are usually introduced in __init__, so the user's code would fail every time A is instantiated, not only in some obscure corner case. It's also somewhat rare to use __slots__ in user code.

It's also somewhat rare to use slots in user code.

This is shocking to me :)
So many people ignore this awesome feature for some reason.

#14611 adds __slots__ to almost all of the standard library. I might do the third-party stubs soon too.

Seems that the discussion has reached a conclusion: __slots__ has been added, and PR #14619 (for third-party stubs) was the final step.

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Consider adding `slots` to stubs? #8832

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Consider adding __slots__ to stubs? #8832

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AlexWaygood commented on Oct 3, 2022

hauntsaninja commented on Oct 3, 2022

JelleZijlstra commented on Oct 3, 2022

sobolevn commented on Oct 3, 2022

Akuli commented on Oct 3, 2022

AlexWaygood commented on Oct 3, 2022

sobolevn commented on Oct 4, 2022

Akuli commented on Oct 4, 2022

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JelleZijlstra commented on Aug 21, 2025

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Consider adding `slots` to stubs? #8832