singledispatchmethod raises an error when relying on a forward declaration

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BPO	41987
Nosy	@gvanrossum, @glyph, @ambv, @ethanhs, @joernheissler, @isidentical, @mental32, @ryansobol, @AlexWaygood
PRs	bpo-41987: Fix unnecessary evaluation of return type forward declaratons in singledispatch. #23216

^{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 = None
created_at = <Date 2020-10-09.20:11:43.528>
labels = ['type-bug', 'library', '3.9', '3.10']
title = 'singledispatchmethod raises an error when relying on a forward declaration'
updated_at = <Date 2022-01-11.09:46:19.220>
user = 'https://github.com/glyph'

bugs.python.org fields:

activity = <Date 2022-01-11.09:46:19.220>
actor = 'AlexWaygood'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-10-09.20:11:43.528>
creator = 'glyph'
dependencies = []
files = []
hgrepos = []
issue_num = 41987
keywords = ['patch']
message_count = 16.0
messages = ['378346', '378350', '378353', '378361', '379896', '379911', '380797', '380799', '380800', '380803', '380832', '380843', '380845', '380852', '409572', '410269']
nosy_count = 9.0
nosy_names = ['gvanrossum', 'glyph', 'lukasz.langa', 'ethan smith', 'joernheissler', 'BTaskaya', 'mental', 'ryansobol', 'AlexWaygood']
pr_nums = ['23216']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41987'
versions = ['Python 3.9', 'Python 3.10']

[glyph]

This example:

from __future__ import annotations
from functools import singledispatchmethod


class Comparable:
 @singledispatchmethod
 def compare(self, arg: object):
 raise NotImplementedError("what")

 @compare.register
 def _(self, arg: Comparable):
 return "somewhat similar"

print(Comparable().compare(Comparable()))

Produces this result:

File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/typing.py", line 518, in _evaluate
eval(self.__forward_code__, globalns, localns),
File "<string>", line 1, in <module>
NameError: name 'Comparable' is not defined

It seems like perhaps singledispatchmethod should defer its type evaluation to its first invocation?

[isidentical]

AFAIK the normal way of registering types (dispatcher.register(<type>)) also requires that registered type be defined at the execution type. I guess, if we are going to support such a thing, we might end up with supporting passing strings into the .register() as the initial argument of matching type to be consistent. Anyways, this would be a feature request for 3.10+, so changing the version info.

[gvanrossum]

This behavior (only relevant with from __future__ import annotations) has been around since @singledispatchmethod was introduced in 3.8, so I agree we should treat it as a feature request.

In the meantime, maybe a workaround is to move the register call out of the class? It looks a little bit ugly but probably works.

(Disclaimer: I'm not familiar with singledispatchmethod.)

[glyph]

The behavior is the same with a traditional quoted forward declaration, so it’s not specific to the __future__ import; I just phrased the example that way to show how it’s going to look in the future and to illustrate how it might crop up in a way which is maximally confusing to users less familiar with the internals of type annotations.

[ryansobol]

It's worth pointing out that a similar error is produced for a forward-referenced return type of a registered method, but only for python3.9. For example:

from __future__ import annotations
from functools import singledispatchmethod


class Integer:
 def __init__(self, value: int):
 self.value = value

 def __str__(self) -> str:
 return str(self.value)

 @singledispatchmethod
 def add(self, other: object) -> Integer:
 raise NotImplementedError(f"Unsupported type {type(other)}")

 @add.register
 def _(self, other: int) -> "Integer":
 return Integer(self.value + other)

print(Integer(2).add(40))

This code runs without error in python3.8, and I am using this technique in code running in a production environment.

$ python3.8 --version
Python 3.8.6
$ python3.8 integer.py
42

However, this code throws a NameError in python3.9.

$ python3.9 --version
Python 3.9.0
$ python3.9 integer.py
Traceback (most recent call last):
 File "/Users/ryansobol/Downloads/integer.py", line 5, in <module>
 class Integer:
 File "/Users/ryansobol/Downloads/integer.py", line 17, in Integer
 def _(self, other: int) -> "Integer":
 File "/usr/local/Cellar/python@3.9/3.9.0_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/functools.py", line 909, in register
 return self.dispatcher.register(cls, func=method)
 File "/usr/local/Cellar/python@3.9/3.9.0_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/functools.py", line 860, in register
 argname, cls = next(iter(get_type_hints(func).items()))
 File "/usr/local/Cellar/python@3.9/3.9.0_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/typing.py", line 1386, in get_type_hints
 value = _eval_type(value, globalns, localns)
 File "/usr/local/Cellar/python@3.9/3.9.0_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/typing.py", line 254, in _eval_type
 return t._evaluate(globalns, localns, recursive_guard)
 File "/usr/local/Cellar/python@3.9/3.9.0_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/typing.py", line 497, in _evaluate
 self.__forward_value__ = _eval_type(
 File "/usr/local/Cellar/python@3.9/3.9.0_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/typing.py", line 254, in _eval_type
 return t._evaluate(globalns, localns, recursive_guard)
 File "/usr/local/Cellar/python@3.9/3.9.0_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/typing.py", line 493, in _evaluate
 eval(self.__forward_code__, globalns, localns),
 File "<string>", line 1, in <module>
NameError: name 'Integer' is not defined

I know that some may see this issue as a feature request for 3.10+. However, for me, it is a bug preventing my code from migrating to 3.9.