`issubclass` and `isinstance` doesn't check for all 2nd argument types

[thatbirdguythatuknownot]

BPO	44932
Nosy	@gvanrossum, @serhiy-storchaka, @Fidget-Spinner, @thatbirdguythatuknownot

^{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 = <Date 2021-08-17.11:57:52.397>
created_at = <Date 2021-08-17.07:26:02.366>
labels = ['type-bug', 'library', '3.9', '3.10', '3.11']
title = "`issubclass` and `isinstance` doesn't check for all 2nd argument types"
updated_at = <Date 2021-08-17.19:09:23.977>
user = 'https://github.com/thatbirdguythatuknownot'

bugs.python.org fields:

activity = <Date 2021-08-17.19:09:23.977>
actor = 'gvanrossum'
assignee = 'none'
closed = True
closed_date = <Date 2021-08-17.11:57:52.397>
closer = 'kj'
components = ['Library (Lib)']
creation = <Date 2021-08-17.07:26:02.366>
creator = 'Crowthebird'
dependencies = []
files = []
hgrepos = []
issue_num = 44932
keywords = []
message_count = 5.0
messages = ['399717', '399722', '399741', '399745', '399783']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'kj', 'Crowthebird']
pr_nums = []
priority = 'normal'
resolution = 'wont fix'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44932'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[thatbirdguythatuknownot]

When I tried using isinstance with a type (e.g. bool) as the 1st argument and a parameterized generic in a tuple (e.g. '(bool, list[bool])') as the 2nd argument, it raised a TypeError, 'isinstance() argument 2 cannot be a parameterized generic'. But when I did the same thing in issubclass, it returned a boolean and did not raise any TypeErrors. Using isinstance with an object as the 1st argument and the same tuple 2nd argument also returned a boolean, and did not raise any TypeErrors. Is this expected behaviour, or should this be fixed? This was tested in Python 3.10.0rc1 in a 64-bit system.

[Fidget-Spinner]

@paul, Steve, Tim and Zach, I've removed y'all from nosy as this isn't a Windows issue (but a typing/builtin types one). If you feel that I shouldn't have done that: my apologies and please do tell me.

To summarize what OP said:

>>> isinstance(bool, (bool, list[bool]))
TypeError: isinstance() argument 2 cannot be a parameterized generic

>>> issubclass(bool, (bool, list[bool]))
True

@op, this seems like a bug. issubclass should raise according to PEP-585. In this case it's short-circuiting so it doesn't check the 2nd arg. Swapping the arguments around gives the expected TypeError.

>>> issubclass(bool, (list[bool], bool))
TypeError: issubclass() argument 2 cannot be a parameterized generic

At the same time, I'm hesitant to run a check through everything in the types tuple first. That would hurt issubclass performance (I'm not sure by how much). What do you think, Guido and Serhiy?