[Python-Dev] PEP 560: bases classes (original) (raw)

Ivan Levkivskyi levkivskyi at gmail.com
Thu Nov 16 17:57:06 EST 2017

On 16 November 2017 at 18:58, Ethan Furman <ethan at stoneleaf.us> wrote:

On 11/16/2017 04:22 AM, Ivan Levkivskyi wrote:

On 16 November 2017 at 07:56, Nick Coghlan wrote:

Jim also raised an important point that needs clarification at the spec

>> level: given multiple entries in "origbases" with mroentries methods, >> do all such methods get passed the same origbases tuple? Or do they receive partially resolved ones, such that bases listed before them have >> already been resolved to their MRO entries by the time they run. Yes, they all get the same initial bases tuple as an argument. Passing > updated ones will cost a bit more and I don't think it will be needed > (in the worst case a base can resolve another base by calling its > mroentries manually). I will clarify this in the PEP. If the extra complexity is to: > - given origbases, 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 And each method is passed the same original tuple (without other methods' updates) then don't we end up in a situation where we can have duplicates base classes?

Not that it is impossible now (in certain sense):

class MultiMeta(type):
    def __new__(mcls, name, bases, ns):
        return super().__new__(mcls, name, (), ns)

class MultiBase(metaclass=MultiMeta):
    pass

class C(MultiBase, list, list, MultiBase, dict, dict, dict):  # OK
    pass

What is probably confusing in the current PEP text, is that it doesn't say clearly that the substitution happens before any other steps in build_class. Therefore all normal checks (like duplicate bases and MRO consistency) happen and e.g.

class C(List[int], List[str]):
    pass

will fail with:

TypeError: duplicate base class list

(by the way while playing with this I have found a bug in the reference implementation)

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