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name Punditsdkoslkdosdkoskdo

What does “mro()” do?

In django.utils.functional.py:

for t in type(res).mro():  # <----- this
    if t in self.__dispatch:
        return self.__dispatch[t][funcname](res, *args, **kw)

I don't understand mro(). What does it do and what does "mro" mean?

Follow along...:

>>> class A(object): pass
... 
>>> A.__mro__
(<class '__main__.A'>, <type 'object'>)
>>> class B(A): pass
... 
>>> B.__mro__
(<class '__main__.B'>, <class '__main__.A'>, <type 'object'>)
>>> class C(A): pass
... 
>>> C.__mro__
(<class '__main__.C'>, <class '__main__.A'>, <type 'object'>)
>>> 

As long as we have single inheritance, __mro__ is just the tuple of: the class, its base, its base's base, and so on up to object (only works for new-style classes of course).

Now, with multiple inheritance...:

>>> class D(B, C): pass
... 
>>> D.__mro__
(<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <type 'object'>)

...you also get the assurance that, in __mro__, no class is duplicated, and no class comes after its ancestors, save that classes that first enter at the same level of multiple inheritance (like B and C in this example) are in the __mro__ left to right.

Every attribute you get on a class's instance, not just methods, is conceptually looked up along the __mro__, so, if more than one class among the ancestors defines that name, this tells you where the attribute will be found -- in the first class in the __mro__ that defines that name.

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mro() stands for Method Resolution Order. It returns a list of types the class is derived from, in the order they are searched for methods.

mro() or __mro__ works only on new style classes. In python 3, they work without any issues. But in python 2 those classes need to inherit from object.

  • 82
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This would perhaps show the order of resolution.

class A(object):
    def dothis(self):
        print('I am from A class')

class B(A):
    pass

class C(object):
    def dothis(self):
        print('I am from C class')

class D(B, C):
    pass

d_instance= D()
d_instance.dothis()
print(D.mro())

and response would be

I am from A class
[<class '__main__.D'>, <class '__main__.B'>, <class '__main__.A'>, <class '__main__.C'>, <class 'object'>]

The rule is depth-first, which in this case would mean D, B, A, C.

Python normally uses a depth-first order when searching inheriting classes, but when two classes inherit from the same class, Python removes the first mention of that class from mro.

  • 10
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      • 1
    • @RuthvikVaila: I think that part is just poorly stated. In this blog post about the history of Python, Guido van Rossum remarks (about the MRO scheme introduced in Python 2.2) "If any class was duplicated in this search, all but the last occurrence would be deleted from the MRO list" (emphasis mine). This implies that it could remove more than just the first "mention" of the class.

Order of resolution will be different in diamond inheritance.

class A(object):
    def dothis(self):
        print('I am from A class')


class B1(A):
    def dothis(self):
        print('I am from B1 class')
    # pass


class B2(object):
    def dothis(self):
        print('I am from B2 class')
    # pass


class B3(A):
    def dothis(self):
        print('I am from B3 class')


# Diamond inheritance
class D1(B1, B3):
    pass


class D2(B1, B2):
    pass


d1_instance = D1()
d1_instance.dothis()
# I am from B1 class
print(D1.__mro__)
# (<class '__main__.D1'>, <class '__main__.B1'>, <class '__main__.B3'>, <class '__main__.A'>, <class 'object'>)


d2_instance = D2()
d2_instance.dothis()
# I am from B1 class
print(D2.__mro__)
# (<class '__main__.D2'>, <class '__main__.B1'>, <class '__main__.A'>, <class '__main__.B2'>, <class 'object'>)
  • 1
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      • 1
    • In the multiple inheritance scenario, any specified attribute is searched first in the current class. If not found, the search continues into parent classes in depth-first, left-right fashion without searching same class twice.
    • sorry but i don't understand why in first case it's go to class B3 but in second case it's go to class A after class B1

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