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I stumbled across this extra, no-underscores mro method when I was using __metaclass__ = abc.ABCMeta. It seems to be the same as __mro__ except that it returns a list instead of a tuple. Here's a random example (ideone snippet):

import abc
import copy

class Life(object):
    __metaclass__ = abc.ABCMeta

    @abc.abstractmethod
    def reproduce(self):
        pass

class Bacterium(Life):
    def reproduce(self):
        return copy.deepcopy(self)

wiggly = Bacterium()

print wiggly.__class__.__mro__
# (<class '__main__.Bacterium'>, <class '__main__.Life'>, <type 'object'>)

print wiggly.__class__.mro()
# [<class '__main__.Bacterium'>, <class '__main__.Life'>, <type 'object'>]

I found later that this isn't unique to ABCMeta but is available in all new-style classes.

So... why? What is this doing that __mro__ isn't?

Air
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1 Answers1

11

Directly from the documentation:

This method can be overridden by a metaclass to customize the method resolution order for its instances. It is called at class instantiation, and its result is stored in _mro_.

So, mro() is called on instantiation and caches its result in __mro__. They don't really have the same purpose.

jeromej
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    So when should I use which? Does the standard implementation of `mro()` use `__mro__` as a cache, in that it does not recompute? If yes, then that's the reason for `__mro__` and should be considered an implementation detail. Otherwise, what's the purpose of it? – burnpanck Nov 20 '14 at 13:45
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    @burnpanck. `__mro__` is explicitly used by `super`: https://docs.python.org/3/reference/datamodel.html#invoking-descriptors – Mad Physicist Feb 01 '18 at 06:27
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    https://github.com/python/cpython/blob/main/Objects/typeobject.c#L2678 Can check this out in the Python CAPI to see how .mro() method is called by the interpreter. – Jim Aug 12 '23 at 08:00