On 2/4/21 11:33 PM, Stefan van der Walt wrote:
The question raised on the GitHub issue [0], "Why can't we make `Type[T]` also work for other special forms?", will likely come up for others reading this too. Would it be worth adding a sentence to explain?
[0] https://github.com/python/mypy/issues/9773#issuecomment-736810721
Guido I tried to write up an explanation for this question, but couldn't come up with a very strong answer. Could you, Ivan, or Jukka (on CC) elaborate why it would be a bad idea to widen `Type` such that typing special forms like `Optional[str]` or `Any` would be assignable to it? My lukewarm attempt to explain the current rationale is at the bottom of this email. Personally I think it really *would* be a lot easier to just widen Type[T] to also include typing special forms. It's unclear to me why it's specifically important to have it only match things that can be used in isinstance(). It doesn't even do that job perfectly at the moment: `isinstance(MyTypedDict(value=1), MyTypedDict)` fails at runtime with TypeError but mypy treats `MyTypedDict` as assignable to `Type` anyway. -- David Foster | Seattle, WA, USA Contributor to TypedDict support for mypy
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Rejected Ideas ============== Widen Type[T] to support all typing special forms ------------------------------------------------- `Type` was [designed] to only be used for things that can be the second argument of `isinstance()`. And those things must be actual class objects -- they cannot be typing special forms like `Any`, `Optional[int]` or `List[str]`. Since `Type` is restricted in this way, it is necessary to introduce a new spelling, `TypeForm`, to include the additional special forms as well. [designed]: https://github.com/python/mypy/issues/8992#issuecomment-643369024 <<<<<<<<<<