[Python-ideas] PEP 560 (second post)

Koos Zevenhoven k7hoven at gmail.com
Fri Nov 10 15:46:49 EST 2017

On Fri, Nov 10, 2017 at 10:26 PM, Ivan Levkivskyi <levkivskyi at gmail.com>

> On 10 November 2017 at 21:19, Koos Zevenhoven <k7hoven at gmail.com> wrote:
>> On Fri, Nov 10, 2017 at 8:33 PM, Ivan Levkivskyi <levkivskyi at gmail.com>
>> wrote:
>>> On 10 November 2017 at 18:39, Koos Zevenhoven <k7hoven at gmail.com> wrote:
>>>> On Wed, Sep 27, 2017 at 12:28 PM, Ivan Levkivskyi <levkivskyi at gmail.com
>>>> > wrote:
>>>>>>>>> After creating the class,
>>>>> the original bases are saved in ``__orig_bases__`` (currently this is
>>>>> also
>>>>> done by the metaclass).
>>>> ​Those are *still* bases, right, even if they are not in the mro?​ I'm
>>>> not sure if this is a naming thing or something even more.
>>> The objects that have __subclass_base__ method (proposed to rename to
>>> __mro_entry__)
>>> are removed from __bases__ attributed of the newly created class.
>>> Otherwise they may cause a metaclass conflict.
>>> One can however still call them syntactic (or static?) bases. For
>>> example this is how it is going to be used by typing:
>>>     from typing import List
>>>     class Tokens(List[int]):
>>>         ...
>>>     assert Tokens.__bases__ == (list,)
>> ​Why is List[int] not allowed to be the base? Neither method-lookup
>> performance nor the metaclass conflict issue seem to depend on whether
>> List[int] is in __bases__.
> The situation is actually quite opposite. Interestingly, the whole
> discussion started from Mark Shannon pointing to these problems with
> List[int] at the Language Summit.
> The original discussion on typing tracker is referenced in the PEP draft.
​What do you mean? I don't see any mention of __bases__ in the discussion.

Perhaps related, if the syntax indeed were like this (as suggested by

class Tokens(list):

​then I would expect List[int] to be in __implements__ or something like

But how to interpret that? Maybe something like "List describes the
interface/API that list has"?

-- Koos

+ Koos Zevenhoven + http://twitter.com/k7hoven +
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