(as instructed in bpo-46244, I've created this ticket)

The subclass EntryPoints defines slots, but its base DeprecatedList does not.

This appears to be a mistake.

If so, I'd like to create a PR to fix it.

Perhaps it is a mistake. The __slots__ were added as a (possible premature) optimization in this conversation.

It's not obvious to me what the danger is in defining __slots__ in a child class but not in a parent. Can you point me to resources or examples that help me understand the concern or missed expectation? The primary motivation behind defining slots is to help ensure immutability. Does the lack of slots violate that expectation?

Convince me of the value of fixing this concern and please feel free to proceed with a PR.

@jaraco thanks for your quick response.

In short: __slots__ allows class layout to be optimized by replacing the class __dict__ with specific descriptors. This results in a class where only specific attributes can be get/set.

However, you only really get the savings from __slots__ if all base classes also implement it.

An example:

    from pympler.asizeof import asizeof  # checks complete size of objects in memory

    class EmptyNoSlots: pass

    class EmptyWithSlots: __slots__ = ()

    class NoSlots:
        def __init__(self, a, b): self.a, self.b = a, b

    class WithSlots:
        __slots__ = ("a", "b")
        def __init__(self, a, b): self.a, self.b = a, b

    print(asizeof(EmptyNoSlots()))    # 152
    print(asizeof(EmptyWithSlots()))  # 32
    print(asizeof(NoSlots(1, 2)))     # 328
    print(asizeof(WithSlots(1, 2)))   # 112

# Let's look at inheritance:

    class WithSlotsAndProperBaseClass(EmptyWithSlots):
        __slots__ = ("a", "b")
        def __init__(self, a, b): self.a, self.b = a, b

    class NoSlotsAtAll(EmptyNoSlots):
        def __init__(self, a, b): self.a, self.b = a, b

    class WithSlotsAndBadBaseClass(EmptyNoSlots):
        __slots__ = ("a", "b")
        def __init__(self, a, b): self.a, self.b = a, b

    print(asizeof(WithSlotsAndProperBaseClass(1, 2)))  # 112
    print(asizeof(NoSlotsAtAll(1, 2)))                 # 328
    print(asizeof(WithSlotsAndBadBaseClass(1, 2)))     # 232 -- oh no!

In importlib:

                   list   \<- has slots (builtin)
   DeprecatedList(list)   \<- no \_\_slots__ defined

EntryPoints(DeprecatedList) <- defines __slots__

Besides the lost memory savings, because DeprecatedList has no slots, you can still do:

EntryPoints().foo = 6  # setting a random attribute, not the intention.

Further reading: https://stackoverflow.com/a/28059785

Today I learned something. Thanks Arie. Yes, I agree that's a mistake. Perhaps the test suite should also have a test to capture the missed expectation (that __dict__ should not be present or setting attributes on EntryPoints instances should fail).

I'm pretty sure both EntryPoints and DeprecatedList were introduced in Python 3.10, so 3.9 and 3.8 aren't relevant.

New changeset dd76b3f by Arie Bovenberg in branch 'main':
bpo-46246: add missing __slots__ to importlib.metadata.DeprecatedList (GH-30452)
dd76b3f

New changeset 1124ab6 by Miss Islington (bot) in branch '3.10':
bpo-46246: add missing __slots__ to importlib.metadata.DeprecatedList (GH-30452)
1124ab6