[Python-ideas] Optional static typing -- the crossroads

[Nick Coghlan]

On 15 August 2014 21:01, Bill Winslow <bunslow at gmail.com> wrote:
> Consider the following:
>
>
>>>> from collections import abc as types
>>>> isinstance(dict, types.Mapping)
> False
>>>> isinstance(types.Mapping, dict)
> False
>>>> isinstance(list, types.MutableSequence)
> False
>>>> isinstance(types.MutableSequence, list)
> False
>>>> isinstance(list, types.Sized)
> False

You're doing instance checks on subclasses - that's never going to
work. Once you account for the type/instance distinction, you can see
everything is correctly registered:

>>> from collections import abc as cabc
>>> issubclass(dict, cabc.Mapping)
True
>>> issubclass(list, cabc.Sequence)
True
>>> issubclass(list, cabc.Sized)
True
>>> isinstance(dict(), cabc.Mapping)
True
>>> isinstance(list(), cabc.Sequence)
True
>>> isinstance(list(), cabc.Sized)
True

(in Python 2, a couple of builtins claim ABCs they don't actually
implement fully, but that's addressed in newer versions of Python 3)

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia