In Python, how do you make a subclass from a superclass?
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).
class Subclass (SuperClass):
# Subclass stuff here
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
class Mammal(object):
#mammal stuff
class Dog(Mammal):
#doggie stuff
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.
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())