More differences in instance and subclass checks between typing.Union and types.Union

[serhiy-storchaka]

BPO	44668
Nosy	@gvanrossum, @serhiy-storchaka, @Fidget-Spinner

^{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 2021-07-18.17:21:30.520>
labels = ['type-bug', 'library', '3.10', '3.11']
title = 'More differences in instance and subclass checks between typing.Union and types.Union'
updated_at = <Date 2021-07-19.17:57:02.931>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2021-07-19.17:57:02.931>
actor = 'gvanrossum'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-07-18.17:21:30.520>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 44668
keywords = []
message_count = 4.0
messages = ['397757', '397775', '397778', '397823']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'kj']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44668'
versions = ['Python 3.10', 'Python 3.11']

Linked PRs

• gh-88834: Unify the instance check for typing.Union and types.UnionType #128363
• [3.13] gh-88834: Unify the instance check for typing.Union and types.UnionType (GH-128363) #128370
• [3.12] gh-88834: Unify the instance check for typing.Union and types.UnionType (GH-128363) #128371

[serhiy-storchaka]

1. Checks for types.Union ignore any non-type args. Checks for typing.Union require fail on first checked non-type (but it can short circuit).

>>> import typing
>>> T = typing.TypeVar('T')
>>> issubclass(int, int | T | str)
True
>>> issubclass(int, str | T | int)
True
>>> issubclass(int, typing.Union[int, T, str])
True
>>> issubclass(int, typing.Union[str, T, int])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/typing.py", line 1208, in __subclasscheck__
    if issubclass(cls, arg):
       ^^^^^^^^^^^^^^^^^^^^
TypeError: issubclass() arg 2 must be a class, a tuple of classes, or a union.
>>> isinstance(1, int | T | str)
True
>>> isinstance(1, str | T | int)
True
>>> isinstance(1, typing.Union[int, T, str])
True
>>> isinstance(1, typing.Union[str, T, int])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/typing.py", line 1204, in __instancecheck__
    return self.__subclasscheck__(type(obj))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/typing.py", line 1208, in __subclasscheck__
    if issubclass(cls, arg):
       ^^^^^^^^^^^^^^^^^^^^
TypeError: issubclass() arg 2 must be a class, a tuple of classes, or a union.

2. __instancecheck__ of typing.Union uses __subclasscheck__ of args instead of __instancecheck__. In normal cases the result should be the same, but there should be a reason of having two different special methods.

[gvanrossum]

This seems a low-priority issue to me. The code is invalid, but I don't think it's very important to raise an exception (typing.Union is also inconsistent). Static type checkers will flag all these as errors. (Or perhaps none of them? It may depend on the meaning of T in the current scope.)

[serhiy-storchaka]

It is easy to fix once we define the correct behavior in corner cases.

[gvanrossum]

Sure.

[AlexWaygood]

This looks to have been fixed at some point, though I don't know when. types.UnionType and typing.Union are now consistent: they will both succeed if they hit a "success case" before they hit an "invalid case", but they will both error out if they hit an invalid case before a success case. On main (and the behaviour is the same on 3.11.2):

>>> import typing
>>> T = typing.TypeVar("T")
>>> issubclass(int, int | T | str)
True
>>> issubclass(int, str | T | int)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1698, in __subclasscheck__
    if issubclass(cls, arg):
       ^^^^^^^^^^^^^^^^^^^^
TypeError: issubclass() arg 2 must be a class, a tuple of classes, or a union
>>> issubclass(int, typing.Union[int, T, str])
True
>>> issubclass(int, typing.Union[str, T, int])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1698, in __subclasscheck__
    if issubclass(cls, arg):
       ^^^^^^^^^^^^^^^^^^^^
TypeError: issubclass() arg 2 must be a class, a tuple of classes, or a union
>>> isinstance(1, int | T | str)
True
>>> isinstance(1, str | T | int)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1694, in __instancecheck__
    return self.__subclasscheck__(type(obj))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1698, in __subclasscheck__
    if issubclass(cls, arg):
       ^^^^^^^^^^^^^^^^^^^^
TypeError: issubclass() arg 2 must be a class, a tuple of classes, or a union
>>> isinstance(1, typing.Union[int, T, str])
True
>>> isinstance(1, typing.Union[str, T, int])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1694, in __instancecheck__
    return self.__subclasscheck__(type(obj))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1698, in __subclasscheck__
    if issubclass(cls, arg):
       ^^^^^^^^^^^^^^^^^^^^
TypeError: issubclass() arg 2 must be a class, a tuple of classes, or a union

Depending on whether it was fixed "on purpose" or "by accident", we may need to add regression tests.

[serhiy-storchaka]

Indeed, the first part was fixed in #91603.

#128363 fixes the second part.