3.13 inheritance

Inheritance means one class can inherit attributes and methods of parent class.

class Human(object):

    def __init__(self, name):
        self.name = name

    def say_salam(self):
        print('Salam, my name is', self.name)


class Muslim(Human):
    pass


ali = Muslim('Ali')
ali.say_salam()

Salam, my name is Ali

The Muslim class itself does not have a method say_salam but since it is inheriting from Human class and say_slam exists in Human class, we do end up calling say_salam method fo Human class.

We can check the type of the ali which is an object and is an instance of class Muslim. There are two ways to verify this.

isinstance(ali, Muslim), type(ali) == Muslim

(True, True)

However, isinstance checks the parent classes as well but type function does not do so.

isinstance(ali, Human), type(ali) == Human

(True, False)

arjun = Human('arjun')

isinstance(arjun, Human), isinstance(arjun, Muslim)

(True, False)

So it is always safe to check the type of an object by instance.

Inheritance can occur between more than two classes as well. In following, Pakistani class inherits from Muslim class which itself inherits from Human class.

class Human(object):

    def __init__(self, name):
        self.name = name


class Muslim(Human):
    pass


class Pakistani(Muslim):
    pass


ali = Pakistani('ali')

isinstance(ali, Human)

True

We can override the methods of parent or super class in its child class by simply redefining the method.

class Human(object):

    def __init__(self, name):
        self.name = name

    def introduction(self):
        print('Hello, my name is', self.name)


class Muslim(Human):

    def introduction(self):
        print('Salam, my name is', self.name)


ali = Muslim("ali")

ali.introduction()

Salam, my name is ali

Human.introduction(ali)

Hello, my name is ali

If in a child class we want to call/use method from a parent/super class, we can call method from parent/super class using the super() function as following.

class Human(object):

    def __init__(self, name):
        self.name = name

    def introduction(self):
        print('Hello, my name is', self.name)


class Muslim(Human):

    def introduction(self):
        print("I will call introduction method of parent class")
        super().introduction()


ali = Muslim("ali")

ali.introduction()

I will call introduction method of parent class
Hello, my name is ali

Above, the introduction method of Muslim class calls the introduction method of Human.

class Human(object):

    def __init__(self, name):
        self.name = name

    def introduction(self):
        print('Hello, my name is', self.name)


class Muslim(Human):

    def introduction(self):
        print("salam: my name is ", self.name)


class Pakistani(Muslim):

    def introduction(self):
        print("I will call introduction method of parent class")
        super().introduction()


ali = Pakistani("ali")

ali.introduction()

I will call introduction method of parent class
salam: my name is  ali

If a method is not present in super / parent class , python will keep on looking in all parent / super classes in the hierarchy until it finds the method or attribute being called.

class Human(object):

    def __init__(self, name):
        self.name = name

    def introduction(self):
        print('Hello, my name is', self.name)


class Muslim(Human):
    pass


class Pakistani(Muslim):

    def introduction(self):
        print("I will call introduction method of parent class")
        super().introduction()


ali = Pakistani("ali")

ali.introduction()

I will call introduction method of parent class
Hello, my name is ali

class Human(object):

    def __init__(self, name):
        self.name = name

    def introduction(self):
        print('Hello, my name is', self.name)


class Muslim(Human):
    pass


class Pakistani(Muslim):
    pass


ali = Pakistani("ali")

ali.introduction()

Hello, my name is ali

However, if the called method does not exist in any of the parent classes, then python will throw AttributeError.

class Human(object):

    def __init__(self, name):
        self.name = name

    def introduction1(self):
        print('Hello, my name is', self.name)


class Muslim(Human):
    pass


class Pakistani(Muslim):
    pass


ali = Pakistani("ali")

# uncomment following line
# ali.introduction()  # AttributeError

If in a child class , we want to implement a method of a parent class but not of immediate parent class, we can directly call that method.

Below, Pakistani class is inheriting from Muslim but in introduction method of Pakistani, we don’t want to call introduction method of Muslim class rather we want to call introduction method of Human class.We can do this by Human.introduction()

class Human(object):

    def __init__(self, name):
        self.name = name

    def introduction(self):
        print('Hello, my name is', self.name)


class Muslim(Human):

    def introduction(self):
        print('Salam, my name is', self.name)


class Pakistani(Muslim):

    def introduction(self):
        Human.introduction(self)
        print("I am a Pakistani. ")


ali = Pakistani("ali")

ali.introduction()

Hello, my name is ali
I am a Pakistani.

One of the function of method overriding is to implement abstract methods.

class Human(object):

    def nationality(self):
        raise NotImplementedError("implement this method in child class")


class Pakistani(Human):

    def nationality(self):
        return "pakistani"


class Iranian(Human):
    def nationality(self):
        return "iranian"


class Israili(Human):
    pass


ali = Pakistani()
ali.nationality()

'pakistani'

armaghan = Iranian()
armaghan.nationality()

'iranian'

yakov = Israili()

# uncomment following line
# yakov.nationality()  # NotImplementedError

We can enlist all base classes in a hierarchy of given class.

class Human(object):
    pass


class Muslim(Human):
    pass


class Pakistani(Human):
    pass


class Punjabi(Pakistani):
    pass


print(Punjabi.__mro__)

(<class '__main__.Punjabi'>, <class '__main__.Pakistani'>, <class '__main__.Human'>, <class 'object'>)

print(Muslim.__mro__)

(<class '__main__.Muslim'>, <class '__main__.Human'>, <class 'object'>)

We can initiate a child class with additional arguments than required to initiate the parent class. For this we have to overwrite __init__() method of the child class.

class Human(object):

    def __init__(self, name):
        self.name = name

    def introduction(self):
        print('Hello, my name is', self.name)


class Muslim(Human):

    def __init__(self, name, sect):
        self.sect = sect
        super(Muslim, self).__init__(name)

    def introduction(self):
        print('Salam, my name is {} and my sect is {}'.format(self.name, self.sect))


# Uncomment following line
# ali = Muslim("ali")  # TypeError

Above, the Muslim class now requires two arguments for initiation i.e. name` and sect. We did not provide value for sect.

ali = Muslim("ali", None)
ali.introduction()

Salam, my name is ali and my sect is None

Instead of using super(ChildClass, self).__init__() we can also simple say super().__init__().

class Human(object):

    def __init__(self, name):
        self.name = name

    def introduction(self):
        print('Hello, my name is', self.name)


class Muslim(Human):

    def __init__(self, name, sect):
        self.sect = sect
        super().__init__(name)

    def introduction(self):
        print('Salam, my name is {} and my sect is {}'.format(self.name, self.sect))


ali = Muslim("ali", None)
ali.introduction()

Salam, my name is ali and my sect is None

type(ali.sect)

kapil = Human('kapil')

# uncomment following line
# kapil.sect  # AttributeError

Since kapil is an instance of Human class which does not have any attribute named sect, thus we could not access this attribute.

multiple-inheritance

A class can also inherit from multiple classes and this is called multiple-inheritance.

class Spirit(object):
    chastity = 'NaN'
    bravery = 'NaN'
    tolerance = 'NaN'


class Body(object):
    weight = 20


class Human(Spirit, Body):
    pass


ali = Human()
print(ali.chastity)

NaN

print(ali.weight)

20

If an attribute of the child class is called, it is first looked in the child class, then in the first parent class and then in second parent class.

class Spirit(object):
    chastity = 'NaN'
    bravery = 'NaN'
    identity = 'NaN'


class Body(object):
    identity = 'name'


class Human(Spirit, Body):
    pass


ali = Human()
print(ali.identity)

NaN

Above, identity attribute exists for Body and Spirit class but since Spirit class comes first, so NaN is printed which is value of identity attribute of Spirit class. Sometimes, we may want to initialize both parent classes with separate initializing arguments. In following, Body class requires two initiating arguments while Spirit class can be initialized with as many arguments as possible.

class Spirit(object):
    def __init__(self, **kwargs):
        print("initializing spirit")
        for k, v in kwargs.items():
            print('setting: ', k, 'to ', v)
            setattr(self, k, v)
        print("finished initializing spirit")


class Body(object):
    def __init__(self, weight, height):
        print("initializing body")
        self.weight = weight
        self.height = height
        print("finished initializing body")


class Human(Spirit, Body):
    def __init__(self, name, weight, height, **kwargs):
        print("initializing Human")
        self.name = name
        Spirit.__init__(self, **kwargs)
        Body.__init__(self, weight, height)


ali = Human('ali', 60, 175, chastity='NotANumber')

print("Name: ", ali.name)
print("Weight: ", ali.weight)
print("Height: ", ali.height)
print("Chastity: ", ali.chastity)

initializing Human
initializing spirit
setting:  chastity to  NotANumber
finished initializing spirit
initializing body
finished initializing body
Name:  ali
Weight:  60
Height:  175
Chastity:  NotANumber

Instead of initializing both parent classes separately, call to super() method will suffice to initiate all parent classes at once.

class Spirit(object):
    def __init__(self, **kwargs):
        print("initializing spirit")
        for k, v in kwargs.items():
            print('setting: ', k, 'to ', v)
            setattr(self, k, v)
        print("finished initializing spirit")


class Body(object):
    def __init__(self, weight, height):
        print("initializing body")
        self.weight = weight
        self.height = height
        print("finished initializing body")


class Human(Spirit, Body):
    def __init__(self, name, weight, height, **kwargs):
        print("initializing Human")
        self.name = name
        super().__init__(weight=weight, height=height, **kwargs)


ali = Human('ali', 60, 175, chastity='NotANumber', honesty="undefined")

print("Name: ", ali.name)
print("Weight: ", ali.weight)
print("Height: ", ali.height)
print("Chastity: ", ali.chastity)

initializing Human
initializing spirit
setting:  weight to  60
setting:  height to  175
setting:  chastity to  NotANumber
setting:  honesty to  undefined
finished initializing spirit
Name:  ali
Weight:  60
Height:  175
Chastity:  NotANumber