[Python-Dev] PEP 484: a "NewType" constructor

Guido van Rossum guido at python.org
Mon Mar 21 12:20:16 EDT 2016

Sorry, for PEP feedback it's best to use this issue in the typing
tracker: https://github.com/python/typing/issues/189 (the issue I
linked to was in the mypy tracker).

On Mon, Mar 21, 2016 at 9:15 AM, Guido van Rossum <guido at python.org> wrote:
> Here's one more thing we'd like to add to PEP 484. The description is
> best gleaned from the issue, in particular
> https://github.com/python/mypy/issues/1284#issuecomment-199021176 and
> following (we're going with option (A)).
> Really brief example:
>   from typing import NewType
>   UserId = NewType('UserId', int)
> Now to the type checker UserId is a new type that's compatible with
> int, but converting an int to a UserId requires a special cast form,
> UserId(x). At runtime UserId instances are just ints (not a subclass!)
> and UserId() is a dummy function that just returns its argument.
> For use cases see the issue. Also send feedback there please.
> --
> --Guido van Rossum (python.org/~guido)

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

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