Convert importlib.abc to use typing.Protocol

[brettcannon]

BPO	38782
Nosy	@warsaw, @brettcannon, @ilevkivskyi, @ZackerySpytz, @nanjekyejoannah
PRs	bpo-38782: Use typing.Protocol in the importlib.abc module #21245

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

Show more details

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-11-12.22:47:40.242>
labels = ['type-feature', 'library', '3.9']
title = 'Convert importlib.abc to use typing.Protocol'
updated_at = <Date 2021-10-20.21:07:21.258>
user = 'https://github.com/brettcannon'

bugs.python.org fields:

activity = <Date 2021-10-20.21:07:21.258>
actor = 'barry'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-11-12.22:47:40.242>
creator = 'brett.cannon'
dependencies = []
files = []
hgrepos = []
issue_num = 38782
keywords = ['patch']
message_count = 1.0
messages = ['356501']
nosy_count = 5.0
nosy_names = ['barry', 'brett.cannon', 'levkivskyi', 'ZackerySpytz', 'nanjekyejoannah']
pr_nums = ['21245']
priority = 'low'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue38782'
versions = ['Python 3.9']

[brettcannon]

Now that typing.Protocol exists we should convert the ABCs in importlib over to protocols as the import system functions off of structural typing and not nominal typing.

[JelleZijlstra]

Protocols are a better conceptual fit for these classes, but wouldn't this change break backwards compatibility? For example, protocols don't support .register().

[brettcannon]

Probably, which is why I figured the protocols would be separate since the ABCs also have default implementations that the protocols wouldn't have (I just forgot to update the issue with my latest thinking 😅). I had not decided if they should be in a separate module or just stick them in importlib.abc.

[brettcannon]

protocols don't support .register()

Doesn't using ABCMeta get Protocol subclasses access to register()?

cpython/Lib/typing.py

Line 2043 in f00512d

class Protocol(Generic, metaclass=_ProtocolMeta):

cpython/Lib/typing.py

Line 1926 in f00512d

class _ProtocolMeta(ABCMeta):

[ilevkivskyi]

Yes, IIRC protocols a supposed to be "drop in" replacement for ABCs, in particular .register() support is tested here

cpython/Lib/test/test_typing.py

Line 3765 in f00512d

def test_protocols_support_register(self):

That said however, most type-checkers don't support .register() well (or at all).

[brettcannon]

That said however, most type-checkers don't support .register() well (or at all).

It would just be for the legacy ABC support. My expectation is the current ABCs will inherit from the protocols, but that most code will type to the protocols themselves and the type checkers will rely on that. But I would still have to either leave the abstract methods that are already there on the ABCs or use @abc.abstractmethod on the protocol classes themselves. Otherwise the protocols might as well be kept separate, in which case it's a question of where to put them in the importlib namespace.

Since I have not written it down, the protocols/types will be for:

• sys.meta_path finders
• sys.path finders
• Loaders
• sys.path_hook callables