Search code examples
pythoninheritancesuperclasssuper

How does `super` interacts with a class's `__mro__` attribute in multiple inheritance?

Today, I read the official doc of super.
In which it mentioned multiple inheritance will be decided by the __mro__ attribute of a class.
So I did a bit experiment, but its result surprised me.

# CODE PART
class GrandFather(object):
    def p(self):
        print "I'm old."

class Father(GrandFather):
    def p(self):
        print "I'm male."

class Mother(object):
    def p(self):
        print "I'm female."

class Son(Father, Mother):
    def p(self):
        print "busy, busy, crwaling. "


 # EXPERIMENT PART
In [1]: Son.__mro__
Out[1]: (__main__.Son, __main__.Father, __main__.GrandFather, __main__.Mother, object)

In [2]: Father.__mro__
Out[2]: (__main__.Father, __main__.GrandFather, object)

In [3]: Mother.__mro__
Out[3]: (__main__.Mother, object)

In [4]: GrandFather.__mro__
Out[4]: (__main__.GrandFather, object)

In [5]: s = Son()

In [6]: super(Son, s).p()
I'm male.

In [7]: super(Father, s).p()
I'm old.

In [8]: super(Mother, s).p()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-8-ce4d0d6ef62d> in <module>()
----> 1 super(Mother, s).p()

AttributeError: 'super' object has no attribute 'p'

In [9]: super(GrandFather, s).p()
I'm female.

Below is part of the official doc I mentioned above, it says:

super(type[, object-or-type])
Return a proxy object that delegates method calls to a parent or sibling class of type.   
This is useful for accessing inherited methods that have been overridden in a class.  
The search order is same as that used by getattr() except that the type itself is skipped.

The __mro__ attribute of the type lists the method resolution search order  
used by both getattr() and super().   
The attribute is dynamic and can change whenever the inheritance hierarchy is updated.

If the second argument is an object, isinstance(obj, type) must be true.

By combining this doc and the result of my experiment. The most confusing part is that when calling with super(GrandFather, s).p() it calls the p() of Mother, but Mother isn't in GrandFather's __mro__, and it is on very inferior order of Son's __mro__.

After a bit pondering. I got a plausible explanation which indicate the incompleteness or deficiency of the official doc:
That is when using with super(type, instance), the super function will search from the __mro__ attribute of the class from who your instance is build, but not the __mro__ attribute of the type you passed to super, even if it satisfied the isinstance(instance, type) condition.

So what happened when you typed super(Class, instance) is:

  1. Python check if isinstance(instance, Class) is True.
  2. Python find the __class__ attribute of instance,
    get the the instance.__class__'s __mro__ attribute.
  3. Python find the index of Class you passed to super in the __mro__ tuple in step2.
  4. Python add the index of step3 by 1, use it to get the corresponding class in __mro__ tuple of step 2, and return the super delegate of this corresponding class.
  5. If the index in step4 exceed length of __mro__ of step2, the delegate of last class in __mro__ of step2 is returned, which is the object class.

Is my understanding right?
If I'm wrong, what's the correct mechanism that super interacts with type's __mro__?
If I'm right, how should I raise an issue for python official doc modification?
Because I think the current version about this item could be misleading.

PS: This test was done by Python 2.7.6 within IPython 3.2.1.

Solution

  • Look at the __mro__ of Son:

    __main__.Son, __main__.Father, __main__.GrandFather, __main__.Mother, object

    According to the doc:

    The __mro__ attribute of the type lists the method resolution search order

    So methods will be searched according to the order in the __mro__ list, from left to right. Call of super(type, instance) will change the starting position to the type specified as the first argument of super() in the __mro__ list of the class of the instance specified as the second argument (if the second argument passed to super is a instance):

    super(Son, s) will proxy to __main__.Father

    super(Father, s) will proxy to __main__.GrandFather

    super(GrandFather, s) will proxy to __main__.Mother

    super(Mother, s) will proxy to object

    The interesting part is why __mro__ of Son is like it is. In other words why Mother is after GrandFather. This is because of how the linearization is working in python:

    the linearization of C is the sum of C plus the merge of the linearizations of the parents and the list of the parents.

    See the examples in the documentation you mentioned, it explains a very similar case.

    So that final result is actually correct: super(GrandFather, s).p() should be I'm female.