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

Peter Ludemann pludemann at google.com
Sun May 15 13:24:08 EDT 2016

On 14 May 2016 at 13:28, Guido van Rossum <gvanrossum at gmail.com> wrote:

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[].

So, should we allow this (by analogy to TypeVar allowing multiple types):

​ ​ def new_user(name: str, user_factory: Type[BasicUser, ProUser]) -> Union[BasicUser, ProUser]:

​ ​ return user_factory(name)

? ​... ​ ​or should it be:

​ def new_user(name: str, ​ ​ user_factory: Type[ ​ Union[ BasicUser, ProUser ​]​ ]) -> Union[BasicUser, ProUser]: return user_factory(name) ​ ?

Although both of these lose the information that the output type is related to the input type; and hopefully the verbosity would encourage people to use TypeVar.

(From these examples, my preference is for Type to be limited to a single choice and anything more complicated should be done with TypeVar, Union, etc.)

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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