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

[Koos Zevenhoven]

For anyone confused about similar things, I expect you to be interested in
my post on python-ideas from today:

https://mail.python.org/pipermail/python-ideas/2017-November/047896.html

––Koos


On Wed, Nov 15, 2017 at 4:20 PM, Jim J. Jewett <jimjjewett at gmail.com> wrote:

> (1)  I found the following (particularly "bases classes") very confusing:
>
> """
> If an object that is not a class object appears in the bases of a class
>
> definition, then ``__mro_entries__`` is searched on it. If found,
> it is called with the original tuple of bases as an argument. The result
> of the call must be a tuple, that is unpacked in the bases classes in place
> of this object. (If the tuple is empty, this means that the original bases
> is
> simply discarded.)
> """
>
> Based on the following GenericAlias/NewList/Tokens example, I think I
> now I understand what you mean, and would have had somewhat less
> difficulty if it were expressed as:
>
> """
> When an object that is not a class object appears in the (tuple of)
> bases of a class
> definition, then attribute ``__mro_entries__`` is searched on that
> non-class object.  If ``__mro_entries__`` found,
> it is called with the entire original tuple of bases as an argument. The
> result
> of the call must be a tuple, which is unpacked and replaces only the
> non-class object in the tuple of bases.  (If the tuple is empty, this
> means that the original bases
> is
> simply discarded.)
> """
>
> Note that this makes some assumptions about the __mro_entries__
> signature that I wasn't quite sure about from the example.  So
> building on that:
>
>     class ABList(A, NewList[int], B):
>
> 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)
>
>     # (D) If there are two non-class objects, I *think* the
>     # second one gets the same arguments as the first,
>     # rather than an intermediate tuple with the first such
>     # object already substituted out.
>
> -jJ
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>



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