[Python-Dev] PEP 560: bases classes / confusion

[Nick Coghlan]

On 16 November 2017 at 00:20, Jim J. Jewett <jimjjewett at gmail.com> wrote:

> I *think* the following will happen:
>
>     "NewList[int]" will be evaluated, and __class_getitem__ called, so
> that the bases tuple will be (A, GenericAlias(NewList, int), B)
>
>     # (A)  I *think* __mro_entries__ gets called with the full tuple,
>     # instead of just the object it is found on.
>     # (B) I *think* it is called on the results of evaluating
>     # the terms within the tuple, instead of the original
>     # string representation.
>     _tmp = __mro_entries__(A, GenericAlias(NewList, int), B)
>
>     # (C)  I *think* __mro_entries__ returns a replacement for
>     # just the single object, even though it was called on
>     # the whole tuple, without knowing which object it
>     # represents.
>     bases = (A, _tmp, B)
>

My understanding of the method signature:

    def __mro_entries__(self, orig_bases):
        ...
        return replacement_for_self

My assumption as to the purpose of the extra complexity was:

- given orig_bases, a method could avoid injecting bases already listed if
it wanted to
- allowing multiple items to be returned provides a way to programmatically
combine mixins without having to define a new subclass for each combination

Cheers,
Nick.

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