[Python-Dev] Adding Type[C] to PEP 484 (original) (raw)

Guido van Rossum gvanrossum at gmail.com
Sat May 14 16:28:46 EDT 2016

On Sat, May 14, 2016 at 11:47 AM, Peter Ludemann <pludemann at google.com> wrote:

I think that Type[C,D] would mean Union[Type[C], Type[D]], but I'm not sure ... I should probably talk to a typing expert about this. (Sorry for thinking out loud; but if we decide that Type[C,D] doesn't make sense, we need to prohibit it)

It sounds iffy enough to prohibit.

I suppose I'm asking: do we allow newuser(Type[BasicUser, ProUser])?

That looks like a call, but Type[] is something to use in the definition.

Here's a trivial example that off the top of my head makes sense:

BasicOrProUser = TypeVar('BasicOrProUser', BasicUser, ProUser) def newuser(name: str, userfactory: Type[BasicOrProuser]) -> BasicOrProUser: return userfactory(name)

That looks reasonable and only has one parameter to Type[].

Or must the Type[...] item have been defined with a TypeVar(..., bound=...), in which case multiple types aren't allowed with Type[...]?

You can use whatever you want. If you want to write Type[int] go right ahead. Even Type[Union[int, str]] should be allowed. There is usually a good reason to use a type variable though -- without one the return type would be the base class. (But then again maybe that could be what you want, or you might not even have a return type, e.g. when the thing just gets thrown into the database.)

--Guido

On 14 May 2016 at 11:30, Guido van Rossum <gvanrossum at gmail.com> wrote: What would Type[C, D] mean?

--Guido (mobile) On May 14, 2016 11:21 AM, "Peter Ludemann via Python-Dev" <_ _python-dev at python.org> wrote:

Is Type[C,D] allowed? Or should multiple types be restricted to TypeVar?

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