Backport NewType as it exists on py310+
#157
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| def __init_subclass__(cls): | ||
| subcls_name = cls.__name__ | ||
| raise TypeError( | ||
| f"Cannot subclass an instance of NewType. " |
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I feel like this would be clearer as
| f"Cannot subclass an instance of NewType. " | |
| f"Cannot subclass a NewType. " |
as it may not be obvious to users that "an instance of NewType" refers to a type. I suppose this is the same error as CPython though, so up to you whether it's worth changing.
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The nice thing about the "an instance of NewType" wording is that it explains why you can't subclass the thing you're trying to subclass (it's an instance, not a class). But maybe the current phrasing is too terse for the explanation to be useful to people who aren't familiar with how NewType works.
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| if sys.version_info >= (3, 10): | ||
| # PEP 604 methods | ||
| # It doesn't make sense to have these methods on Python <3.10 |
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We could allow this in theory, but it would probably be more confusing than useful to users to allow | on only a small subset of types.
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On the other hand:
Python 3.7.16 (default, Dec 26 2022, 20:28:00)
[Clang 14.0.0 (clang-1400.0.29.202)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing_extensions import Self
>>> Self | int
typing.Union[typing_extensions.Self, int]
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On the other hand:
Python 3.7.16 (default, Dec 26 2022, 20:28:00) [Clang 14.0.0 (clang-1400.0.29.202)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> from typing_extensions import Self >>> Self | int typing.Union[typing_extensions.Self, int]
Hahaha that seems wrong though :)
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I guess there's no harm in implementing the methods on Python <3.10, but it feels really weird doing an ad-hoc backport of PEP 604 based on the classes we happen to be reimplementing in typing_extensions.
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Seems like we don't backport PEP 604 for TypeVar or ParamSpec:
Python 3.7.16 (default, Jan 17 2023, 16:06:28) [MSC v.1916 64 bit (AMD64)] :: Anaconda, Inc. on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import typing_extensions
>>> typing_extensions.TypeVar("T") | int
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for |: 'TypeVar' and 'type'
>>> typing_extensions.ParamSpec("P") | int
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for |: 'ParamSpec' and 'type'There was a problem hiding this comment.
Yes, let's leave this as you wrote it. | works on objects that use typing_extensions._SpecialForm on 3.7-3.9, which is arguably wrong but changing it would break compatibility, so let's just leave it as is.
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Agreed that it's not worth it to remove __(r)or__ for objects where we've already accidentally backported PEP 604!
Fixes #156. The implementation and tests are basically just copied from CPython, with a few tweaks