[Python-Dev] PEP 484 wishes

Guido van Rossum guido at python.org
Mon May 18 17:05:14 CEST 2015

On Mon, May 18, 2015 at 12:14 AM, Alex Grönholm <alex.gronholm at nextday.fi>

> 18.05.2015, 02:50, Guido van Rossum kirjoitti:
>  On Sun, May 17, 2015 at 3:07 PM, Alex Grönholm <alex.gronholm at nextday.fi>
> wrote:
>>  Looking at PEP 484, I came up with two use cases that I felt were not
>> catered for:
>>    1. Specifying that a parameter should be a subclass of another
>>    (example: Type[dict] would match dict or OrderedDict; plain "Type" would
>>    equal "type" from builtins)
>  I don't understand. What is "Type"? Can you work this out in a full
> example? This code is already okay:
>  def foo(a: dict):
>     ...
>  foo(OrderedDict())
> This code is passing an *instance* of OrderedDict. But how can I specify
> that foo() accepts a *subclass* of dict, and not an instance thereof?
> A full example:
> def foo(a: Type[dict]):
>     ...
> foo(dict)  # ok
> foo(OrderedDict)  # ok
> foo({'x': 1})  # error

You want the argument to be a *class*. We currently don't support that
beyond using 'type' as the annotation. We may get to this in a future
version; it is relatively uncommon. As to what notation to use, perhaps it
would make more sense to use Class and Class[dict], since in the world of
PEP 484, a class is a concrete thing that you can instantiate, while a type
is an abstraction used to describe the possible values of a

Also, what you gave is still not a full example, since you don't show what
you are going to do with that type. Not every class can be easily
instantiated (without knowing the specific signature). So if you were
planning to instantiate it, perhaps you should use Callable[..., dict] as
the type instead. (The ellipsis is not yet supported by mypy --
https://github.com/JukkaL/mypy/issues/393 -- but it is allowed by the PEP.)

>>    1. Specifying that a callable should take at least the specified
>>    arguments but would not be limited to them: Callable[[str, int, ...], Any]
>> Case #2 works already (Callable[[str, int], Any] if the unspecified
>> arguments are optional, but not if they're mandatory. Any thoughts?
> For #2 we explicitly debated this and found that there aren't use cases
> known that are strong enough to need additional flexibility in the args of
> a callable. (How is the code calling the callable going to know what
> arguments are safe to pass?) If there really is a need we can address in a
> future revision.
> Consider a framework where a request handler always takes a Request object
> as its first argument, but the rest of the arguments could be anything. If
> you want to only allow registration of such callables, you could do this:
> def calculate_sum(request: Request, *values):
>    return sum(values)
> def register_request_handler(handler: Callable[[Request, ...], Any]):
>    ...

Hm... Yeah, you'd be stuck with using Callable[..., Any] for now. Maybe in
a future version of the PEP. (We can't boil the ocean of typing in one PEP.

--Guido van Rossum (python.org/~guido)
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