Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: Ian Kelly <ian.g.kelly@gmail.com>
Newsgroups: comp.lang.python
Subject: PEP 484 stubs with generic types
Date: Tue, 17 Nov 2015 10:49:00 -0700
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Xref: csiph.com comp.lang.python:98939

Playing around a bit with PEP 484, I annotated a function that returns
an asyncio.Future:

import asyncio

def get_future() -> asyncio.Future[int]:
    future = asyncio.Future()
    future.set_result(42)
    return future

The problem with this is that in Python 3.5, asyncio.Future can't be
used as a generic type. Fortunately, the typeshed repository provides
a stub that defines it as a generic type:

https://github.com/python/typeshed/blob/master/stdlib/3.4/asyncio/futures.pyi

This is fine for running a static type checker, but it still creates a
problem when actually running the code. The real asyncio.Future
doesn't support indexing, and so the annotation causes a TypeError at
runtime.

I could write the annotation as a forward reference, avoiding the TypeError:

def get_future() -> "asyncio.Future[int]":
    ...

But PEP 484 stipulates that a forward reference "should evaluate
without errors once the module has been fully loaded", so this is
invalid.

Is there any advice for this case? How are generic types meant to be
used with stub files?