Allow subclassing Any at runtime

[hauntsaninja]

BPO	46998
Nosy	@gvanrossum, @JelleZijlstra, @hauntsaninja, @Fidget-Spinner, @AlexWaygood
PRs	bpo-46998: Allow subclassing Any at runtime #31841

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2022-04-05.02:35:58.785>
created_at = <Date 2022-03-13.03:06:37.673>
labels = ['library', '3.11']
title = 'Allow subclassing Any at runtime'
updated_at = <Date 2022-04-05.02:35:58.784>
user = 'https://github.com/hauntsaninja'

bugs.python.org fields:

activity = <Date 2022-04-05.02:35:58.784>
actor = 'JelleZijlstra'
assignee = 'none'
closed = True
closed_date = <Date 2022-04-05.02:35:58.785>
closer = 'JelleZijlstra'
components = ['Library (Lib)']
creation = <Date 2022-03-13.03:06:37.673>
creator = 'hauntsaninja'
dependencies = []
files = []
hgrepos = []
issue_num = 46998
keywords = ['patch']
message_count = 3.0
messages = ['415023', '416720', '416721']
nosy_count = 5.0
nosy_names = ['gvanrossum', 'JelleZijlstra', 'hauntsaninja', 'kj', 'AlexWaygood']
pr_nums = ['31841']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue46998'
versions = ['Python 3.11']

[hauntsaninja]

Discussed on typing-sig at https://mail.python.org/archives/list/[email protected]/thread/GULRKYI7XOB3FLAEFC6OYSTBS5FIA5PU/

[JelleZijlstra]

New changeset 5a4973e by Shantanu in branch 'main':
bpo-46998: Allow subclassing Any at runtime (GH-31841)
5a4973e

[JelleZijlstra]

Thanks for the idea and patch!

[superbobry]

I understand the rationale for this change, but it does complicate the type system in unspecified (and not fully understood ways). For example

• What does it mean to subclass Any in a) a generic class b) a protocol c) a typing.NamedTuple d) a typing.TypedDict?
• Are Any subclasses plain subtypes of Any? Does this question even make sense?
• How do Any subclasses interact with Any? Are they consistent with Any? Are they consistent subtypes of Any?
• etc

I know it is too late to revert this in 3.11, but is there a chance we could revisit the decision to make Any subclassable and put together a deprecation plan, where this behavior ultimately is going away?

[JelleZijlstra]

We can always decide to change our mind and deprecate this feature, but that's putting the cart before the horse. Type checkers must support Any as a base class (it will come up if you inherit from a class in an untyped library), so the concept needs to be part of the type system somehow. Formalizing what subclassing Any means could be useful, but existing type checkers manage without it.

[superbobry]

Is there a better forum to continue this discussion?

I'm not sure I understand why inheriting from Any is necessary in the type system. In my mind, inheriting from a class in an untyped library is very different. An untyped class is syntactically known to be a class, i.e. a static type, whereas Any is a gradual type.

[AlexWaygood]

Is there a better forum to continue this discussion?

@superbobry I'm a little confused here, because this was a change to runtime behaviour, but your concern seems to be with whether type checkers should allow Any to be subclassed or not. Whether it's allowed at runtime or not is pretty tangential to that discussion -- there are lots of things that are allowed at runtime that aren't allowed by some or all type checkers, and that's how it should be. If you want to write a type checker that disallows Any from ever being subclassed, it's your right to do so (though it's not one that most type checkers have taken), but the decision we took here to change the runtime behaviour has little bearing on that.

Nonetheless, if you do want to argue that we should standardise the behaviour of type checkers so that they always disallow subclassing Any, feel free to open an issue at https://github.com/python/typing or to start a thread on the typing-sig mailing list (https://mail.python.org/archives/list/[email protected]/).

[superbobry]

Thanks for chiming in @AlexWaygood!

My concern is that this change does not just affect the runtime behavior as type checkers will now observe Any subclasses in the source and will need to decide what to do with them.

I will follow up on typing-sig@.

[kmoffett]

I just discovered this behavior change recently, when our codebase upgraded to python 3.11. I would have expected Any to perhaps be a GenericAlias whose __mro_entries__ method returns (object,) or something like that, no? That way, the "subclasses" of Any are still just subclasses of object.

But also, an object can get the same behavior of inheriting from Any by implementing __getattr__ that returns Any, right? And I assume all the mock-like objects already do that, no?