[New-bugs-announce] [issue41987] singledispatchmethod raises an error when relying on a forward declaration

[Glyph Lefkowitz]

New submission from Glyph Lefkowitz <glyph at twistedmatrix.com>:

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?

----------
components: Library (Lib)
messages: 378346
nosy: glyph
priority: normal
severity: normal
stage: needs patch
status: open
title: singledispatchmethod raises an error when relying on a forward declaration
type: behavior
versions: Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41987>
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