Introspection support for typing.overload

[JelleZijlstra]

BPO	46821
Nosy	@gvanrossum, @JelleZijlstra, @sobolevn, @Fidget-Spinner, @AlexWaygood
Superseder	bpo-45100: Improve help() by making typing.overload() information accessible at runtime

^{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 = 'https://github.com/JelleZijlstra'
closed_at = <Date 2022-02-21.20:23:36.411>
created_at = <Date 2022-02-21.19:58:17.232>
labels = ['type-feature', 'library', '3.11']
title = 'Introspection support for typing.overload'
updated_at = <Date 2022-02-21.20:23:36.410>
user = 'https://github.com/JelleZijlstra'

bugs.python.org fields:

activity = <Date 2022-02-21.20:23:36.410>
actor = 'JelleZijlstra'
assignee = 'JelleZijlstra'
closed = True
closed_date = <Date 2022-02-21.20:23:36.411>
closer = 'JelleZijlstra'
components = ['Library (Lib)']
creation = <Date 2022-02-21.19:58:17.232>
creator = 'JelleZijlstra'
dependencies = []
files = []
hgrepos = []
issue_num = 46821
keywords = []
message_count = 3.0
messages = ['413671', '413675', '413676']
nosy_count = 5.0
nosy_names = ['gvanrossum', 'JelleZijlstra', 'sobolevn', 'kj', 'AlexWaygood']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '45100'
type = 'enhancement'
url = 'https://bugs.python.org/issue46821'
versions = ['Python 3.11']

[JelleZijlstra]

Currently, the implementation of @overload (

cpython/Lib/typing.py

Line 2211 in 59585d6

return _overload_dummy

) simply returns a dummy function and throws away the decorated function. This makes it virtually impossible for type checkers using the runtime function object to find overloads specified at runtime.

In pyanalyze, I worked around this by providing a custom @overload decorator, working something like this:

_overloads: dict[str, list[Callable]] = {}

def _get_key(func: Callable) -> str:
    return f"{func.__module__}.{func.__qualname__}"

def overload(func):
    key = _get_key(func)
    _overloads.setdefault(key, []).append(func)
    return _overload_dummy

def get_overloads_for(func):
    key = _get_key(func)
    return _overloads.get(key, [])

A full implementation will need more error handling.

I'd like to add something like this to typing.py so that other tools can also use this information.

[AlexWaygood]

Discussion of similar ideas in bpo-45100

[JelleZijlstra]

Thanks! Closing this as a duplicate.