Confusing error message when attempting to pass a newtype as a match pattern

[not-my-profile]

from typing import NewType

T = NewType('T', str)

match 'test':
 case T(): ...

fails with:

Traceback (most recent call last):
 File "/tmp/foo.py", line 6, in <module>
 case T(): ...
 ^^^
TypeError: called match pattern must be a type

The error message is confusing because T is a type. The error message should probably say class instead of type, since as per the specification it's a class pattern.

Linked PRs

• gh-100530: Change the error message for MatchClass #103576

[AlexWaygood]

Cc. @brandtbucher for pattern-matching

[brandtbucher]

Hm... I'm not really sure there's anything to do here. I think the main issue is Python's general overloading of the word "type" to refer to both static types (for annotations) and types of objects. It's this second group that we're checking for here:

>>> import typing
>>> T = typing.NewType("T", str)
>>> isinstance(T, type)
False

With that said, I'm open to changing the wording of the error message from "type" to "class" if others agree that the current wording is confusing.

[sobolevn]

I agree that class is a much better term here than type. Because, here's one more example where type is not quite correct:

>>> import typing
>>> A = typing.Annotated[str, 'meta']
>>> match 'abc':
... case A(): ...
... 
Traceback (most recent call last):
 File "<stdin>", line 2, in <module>
TypeError: called match pattern must be a type

typing.Annotated[str, 'meta'] is a type. But it is not a class.
Moreover, the ast node is also called MatchClass which is another argument for the wording change.