Python: How do I make a subclass from a superclass?

Question

In Python, how do you make a subclass from a superclass?

Answer 1

# Initialize using Parent
#
class MySubClass(MySuperClass):
    def __init__(self):
        MySuperClass.__init__(self)

Or, even better, the use of Python's built-in function, super() (see the Python 2/Python 3 documentation for it) may be a slightly better method of calling the parent for initialization:

# Better initialize using Parent (less redundant).
#
class MySubClassBetter(MySuperClass):
    def __init__(self):
        super(MySubClassBetter, self).__init__()

Or, same exact thing as just above, except using the zero argument form of super(), which only works inside a class definition:

class MySubClassBetter(MySuperClass):
    def __init__(self):
        super().__init__()

Answer 2

A heroic little example:

class SuperHero(object): #superclass, inherits from default object
    def getName(self):
        raise NotImplementedError #you want to override this on the child classes

class SuperMan(SuperHero): #subclass, inherits from SuperHero
    def getName(self):
        return "Clark Kent"

class SuperManII(SuperHero): #another subclass
    def getName(self):
       return "Clark Kent, Jr."

if __name__ == "__main__":
    sm = SuperMan()
    print(sm.getName())
    sm2 = SuperManII()
    print(sm2.getName())
    

Answer 3

class MySubClass(MySuperClass):
    def __init__(self):
        MySuperClass.__init__(self)

        # <the rest of your custom initialization code goes here>

The section on inheritance in the python documentation explains it in more detail

Answer 4

class Class1(object):
    pass

class Class2(Class1):
    pass

Class2 is a sub-class of Class1

Answer 5

In the answers above, the super is initialized without any (keyword) arguments. Often, however, you would like to do that, as well as pass on some 'custom' arguments of your own. Here is an example which illustrates this use case:

class SortedList(list):
    def __init__(self, *args, reverse=False, **kwargs):
        super().__init__(*args, **kwargs)       # Initialize the super class
        self.reverse = reverse
        self.sort(reverse=self.reverse)         # Do additional things with the custom keyword arguments

This is a subclass of list which, when initialized, immediately sorts itself in the direction specified by the reverse keyword argument, as the following tests illustrate:

import pytest

def test_1():
    assert SortedList([5, 2, 3]) == [2, 3, 5]

def test_2():
    SortedList([5, 2, 3], reverse=True) == [5, 3, 2]

def test_3():
    with pytest.raises(TypeError):
        sorted_list = SortedList([5, 2, 3], True)   # This doesn't work because 'reverse' must be passed as a keyword argument

if __name__ == "__main__":
    pytest.main([__file__])

Thanks to the passing on of *args to super, the list can be initialized and populated with items instead of only being empty. (Note that reverse is a keyword-only argument in accordance with PEP 3102).

Answer 6

There is another way to make subclasses in python dynamically with a function type():

SubClass = type('SubClass', (BaseClass,), {'set_x': set_x})  # Methods can be set, including __init__()

You usually want to use this method when working with metaclasses. When you want to do some lower level automations, that alters way how python creates class. Most likely you will not ever need to do it in this way, but when you do, than you already will know what you are doing.

Answer 7

class Subclass (SuperClass):
      # Subclass stuff here

Answer 8

You use:

class DerivedClassName(BaseClassName):

For details, see the Python docs, section 9.5.

Answer 9

class Mammal(object): 
#mammal stuff

class Dog(Mammal): 
#doggie stuff

Answer 10

Subclassing in Python is done as follows:

class WindowElement:
    def print(self):
        pass

class Button(WindowElement):
    def print(self):
        pass

Here is a tutorial about Python that also contains classes and subclasses.

Answer 11

class BankAccount:

  def __init__(self, balance=0):
    self.balance = int(balance)

  def checkBalance(self): ## Checking opening balance....
    return self.balance

  def deposit(self, deposit_amount=1000): ## takes in cash deposit amount and updates the balance accordingly.
    self.deposit_amount = deposit_amount
    self.balance += deposit_amount
    return self.balance

  def withdraw(self, withdraw_amount=500): ## takes in cash withdrawal amount and updates the balance accordingly
    if self.balance < withdraw_amount: ## if amount is greater than balance return `"invalid transaction"`
        return 'invalid transaction'
    else:
      self.balance -= withdraw_amount
      return self.balance


class MinimumBalanceAccount(BankAccount): #subclass MinimumBalanceAccount of the BankAccount class

    def __init__(self,balance=0, minimum_balance=500):
        BankAccount.__init__(self, balance=0)
        self.minimum_balance = minimum_balance
        self.balance = balance - minimum_balance
        #print "Subclass MinimumBalanceAccount of the BankAccount class created!"

    def MinimumBalance(self):
        return self.minimum_balance

c = BankAccount()
print(c.deposit(50))
print(c.withdraw(10))

b = MinimumBalanceAccount(100, 50)
print(b.deposit(50))
print(b.withdraw(10))
print(b.MinimumBalance())

Answer 12

this is a small code:

# create a parent class

class Person(object):
    def __init__(self):
        pass

    def getclass(self):
        return 'I am a Person'
# create two subclass from Parent_class

class Student(Person):
    def __init__(self):
        super(Student, self).__init__()

    def getclass(self):
        return 'I am a student'


class Teacher(Person):
    def __init__(self):
        super(Teacher, self).__init__()

    def getclass(self):
        return 'I am a teacher'


person1 = Person()
print(person1.getclass())

student1 = Student()
print(student1.getclass())

teacher1 = Teacher()
print(teacher1.getclass())

show result:

I am a Person
I am a student
I am a teacher

Answer 13

A minor addition to @thompsongunner's answer.

To pass args to your superclass (parent), just use the function signature of the parent class:

class MySubClassBetter(MySuperClass):
    def __init__(self, someArg, someKwarg="someKwarg"):
        super().__init__(someArg, someKwarg=someKwarg)

You are calling the parent's __init__() method as if you are constructing any other class which is why you don't need to include self.