Right. I still have no good intuition for what Type[BasicUser, ProUser] would mean so I think you should be required to use the Union, which is clear. Type[] should only allow one parameter.
Most real uses of Type[] use a type variable anyway. I really recommend that you read through https://github.com/python/typing/issues/107 even though it's long and meanders a bit; it comes to the right conclusion in the end. On Sun, May 15, 2016 at 10:24 AM, Peter Ludemann <pludem...@google.com> wrote: > > > On 14 May 2016 at 13:28, Guido van Rossum <gvanros...@gmail.com> wrote: > >> On Sat, May 14, 2016 at 11:47 AM, Peter Ludemann <pludem...@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 new_user(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 new_user(name: str, user_factory: Type[BasicOrProuser]) -> >>> BasicOrProUser: >>> return user_factory(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 <gvanros...@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@python.org> wrote: >>>> >>>>> Is Type[C,D] allowed? Or should multiple types be restricted to >>>>> TypeVar? >>>>> >>>>> _______________________________________________ >>>>> Python-Dev mailing list >>>>> Python-Dev@python.org >>>>> https://mail.python.org/mailman/listinfo/python-dev >>>>> Unsubscribe: >>>>> https://mail.python.org/mailman/options/python-dev/guido%40python.org >>>>> >>>>> >>> >> >> >> -- >> --Guido van Rossum (python.org/~guido) >> > > -- --Guido van Rossum (python.org/~guido)
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