What does "mro()" do?

Question

What does mro() do?

---

Example from django.utils.functional:

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

Answer 1

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() and __mro__ work only on new style classes. In Python 3, they work without any issues. In Python 2, however, those classes need to inherit from object.

Answer 2

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())

The output 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 the MRO.

Answer 3

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'>)

Answer 4

For @stryker 's example, the C3 algorithm is:

L[D(B,C)] = D + merge(BAo, Co, BC)
          = D + B + merge(Ao, Co, C)
          = DB + A + merge(o, Co, C)
          = DBA + merge(Co, C)
          = DBACo

See The Python 2.3 Method Resolution Order | Python.org