bpo-40336: Refactor typing._SpecialForm #19620
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serhiy-storchaka
requested review from
gvanrossum and
ilevkivskyi
as code owners
| isinstance(args[0], str) and | ||
| isinstance(args[1], tuple)): | ||
| # Close enough. | ||
| raise TypeError(f"Cannot subclass {cls!r}") |
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It would be good to keep a custom error message, but not important.
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You are right, I tested again class A(Any): pass, and got "__init__() takes 2 positional arguments but 4 were given" which does not look user friendly.
But the former message was "Cannot subclass <class 'typing._SpecialForm'>" which does not look correct, because we subclass an instance of _SpecialForm, not _SpecialForm itself. It is not possible to fix in __new__, but we can raise better error in custom __mro_entries__.
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Using __mro_entries__ is a great idea!
Lib/typing.py
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| def __repr__(self): | ||
| return 'typing.' + self._name | ||
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| def __reduce__(self): | ||
| return self._name | ||
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| def __call__(self, *args, **kwds): | ||
| raise TypeError(f"Cannot instantiate {self!r}") |
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Again, a custom exception message may be helpful, but not important.
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Your are right. "Cannot instantiate typing.Any" looks better than "'_SpecialForm' object is not callable". Initially I renamed _SpecialForm to special form, but later reverted this change.
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Perfect! Thanks for fixing this.
| # There is no '_type_check' call because arguments to Literal[...] are | ||
| # values, not types. | ||
| return _GenericAlias(self, parameters) | ||
| raise TypeError(f"{self} is not subscriptable") |
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Splitting this is really great. I always wanted to do this, but didn't find a way to do this that wouldn't require boilerplate code, it looks like you have found such way.
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I'll leave this to Ivan -- I am busy with the PEG parser integration. |
https://bugs.python.org/issue40336