[New-bugs-announce] [issue34498] Python 3.7 breaks on singledispatch_function.register(pseudo_type), which Python 3.6 accepted

[Dutcho]

New submission from Dutcho <dutcho at ziggo.nl>:

In Python 3.6, the argument to the register() method of a single-dispatch function (i.e. a function decorated by @functools.singledispatch) can be a 'type-like' class (pseudo-type?) from e.g. the typing module that supports isinstance().

The below demonstrates it works in Python 3.6:

$ py -3.6
Python 3.6.6 (v3.6.6:4cf1f54eb7, Jun 27 2018, 03:37:03) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from functools import singledispatch
>>> from typing import Sequence
>>> isinstance([1,2,3], Sequence) # 'pseudo-type' okay with isinstance
True
>>> @singledispatch
... def f(arg):
...     print('unqualified', arg)
...
>>> @f.register(Sequence) # 'pseudo-type' okay with register
... def _(arg):
...     print('sequence', *arg)
...
>>> f(1)
unqualified 1
>>> f([1,2,3])
sequence 1 2 3

This code breaks in Python 3.7:

$ py
Python 3.7.0 (v3.7.0:1bf9cc5093, Jun 27 2018, 04:59:51) [MSC v.1914 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from functools import singledispatch
>>> from typing import Sequence
>>> isinstance([1,2,3], Sequence) # 'pseudo-type' okay with isinstance
True
>>> @singledispatch
... def f(arg):
...     print('unqualified', arg)
...
>>> @f.register(Sequence)  # 'pseudo-type' NOT okay with register
... def _(arg):
...     print('sequence', *arg)
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "D:\Program Files\Python\Python37\lib\functools.py", line 801, in register
    f"Invalid first argument to `register()`: {cls!r}. "
TypeError: Invalid first argument to `register()`: typing.Sequence. Use either `@register(some_class)` or plain `@register` on an annotated function.

While agreeing a check on the register() arg IS required, the current check isinstance(cls, type) seems overly restrictive. This is especially true when considering the (welcome!) Python 3.7 use of annotations for single dispatch functions.

----------
components: Library (Lib)
messages: 324061
nosy: Dutcho
priority: normal
severity: normal
status: open
title: Python 3.7 breaks on singledispatch_function.register(pseudo_type), which Python 3.6 accepted
type: crash
versions: Python 3.7

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34498>
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