Add text about Type[C]. by gvanrossum · Pull Request #218 · python/typing (original) (raw)

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This addresses #107 (but it doen't close it because the issue also
calls for an implementation in typing.py).

Guido van Rossum added 2 commits

May 13, 2016 12:27

This addresses #107 (but it doen't close it because the issue also calls for an implementation in typing.py).

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I think "Meta-types" is not a good name for this section for four reasons:

1. This could add to the classes vs. types confusion: Unlike metaclasses that are quite different from classes, types do not have such hierarchy, since one should not type annotate types, one can only type annotate classes.
2. Type[C] has normal semantics of type: This is just a set of objects that respond True to issubclass(objc, C).
3. I think someone who is looking for a documentation of this feature will not look for "Meta-types", therefore I would propose to change this to a simpler title, like "Type annotating class objects".
4. Finally, using such terminology could lead to proliferation of "meta": If we call annotations of values that are class objects by Type[C] or by ABCMeta meta-types, then how are we supposed to call annotations of values that are metaclasses (by Type[ABCMeta] for example)? Meta-meta-types? :-)

Miscellaneous things:

• Type[T] should likely be covariant, as discussed before.
• Treatment of __init__ signatures is going to be nontrivial, as discussed in the issue. Perhaps mention that may require a future PEP update? In particular, we discussed the variant Type[[A, ...], C] which would be like Type[C] but with __init__ constrained to supporting positional arguments A, .... This would be an alternative for Callable that is also a type object. Is this potentially going to be included in phase 2?

• Replaced section heading with "The type of class objects" and changed the opening sentence to match.

• Drop "at runtime".

• Replaced "is not allowed" with "is rejected by the type checker" (and then some).

• Clarified that you can write Type[Union[...]] or Type[Any].

• Mentioned that a future PEP revision may deal with constructor signatures.

• Clarified behavior when Type[Any] is used.

• Clarify that Type[T] is covariant (though I'm not sure I understand the concept well enough here to explain it well -- by contrast, if Type[T] were invariant, what would change?).

Sent another omnibus update; please review!

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As I understand, you mean here "a union of classes", since in general Union can accept Tuple, Callable etc. If yes, then maybe it is better to mention this?

It looks good to me. I have no further comments.

Looks good to me as well.