Multiple Inheritance

Multiple inheritance is possible in Python unlike other programming languages. A class can be derived from more than one base classes. The syntax for multiple inheritance is similar to single inheritance.

Python Multiple Inheritance Example

class Base1:
    pass

class Base2:
    pass

class MultiDerived(Base1, Base2):
    pass

The class MultiDerived inherits from both Base1 and Base2.

Multilevel Inheritance in Python

On the other hand, we can inherit form a derived class. This is also called multilevel inheritance. Multilevel inheritance can be of any depth in Python. An example with corresponding visualization is given below.

class Base:
    pass

class Derived1(Base):
    pass

class Derived2(Derived1):
    pass

Method Resolution Order in Python

Every class in Python is derived from the class object. It is the most base type in Python. So technically, all other class, either built-in or user-defines, are derived classes and all objects are instances of object class.

>>> issubclass(list,object)
True
>>> isinstance(5.5,object)
True
>>> isinstance("Hello",object)
True

 

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. So, in the above example of MultiDerived class the search order is [MultiDerived, Base1, Base2, object]. This order is also called linearization of MultiDerived class and the set of rules used to find this order is called Method Resolution Order (MRO). MRO must prevent local precedence ordering and also provide monotonicity. It ensures that a class always appears before its parents and in case of multiple parents, the order is same as tuple of base classes.

MRO of a class can be viewed as the __mro__ attribute or mro() method. The former returns a tuple while latter returns a list.

>>> MultiDerived.__mro__
(<class '__main__.MultiDerived'>,
 <class '__main__.Base1'>,
 <class '__main__.Base2'>,
 <class 'object'>)

>>> MultiDerived.mro()
[<class '__main__.MultiDerived'>,
 <class '__main__.Base1'>,
 <class '__main__.Base2'>,
 <class 'object'>]

Here is a little more complex multiple inheritance example and its visualization along with the MRO.

class X: pass
class Y: pass
class Z: pass

class A(X,Y): pass
class B(Y,Z): pass

class M(B,A,Z): pass

print(M.mro())

Output

[<class '__main__.M'>, <class '__main__.B'>, <class '__main__.A'>, <class '__main__.X'>, <class '__main__.Y'>, <class '__main__.Z'>, <class 'object'>]

Refer to this, for further discussion on MRO and to know the actual algorithm how it is calculated.