Python Method overriding, does signature matter?

by

In Python, methods are just key-value pairs in the dictionary attached to the class. When you are deriving a class from a base class, you are essentially saying that method name will be looked into first derived class dictionary and then in the base class dictionary. In order to “override” a method, you simply re-declare the method in the derived class.

So, what if you change the signature of the overridden method in the derived class? Everything works correctly if the call is on the derived instance but if you make the call on the base instance, you will get an error because the base class uses a different signature for that same method name.

There are however frequent scenarios where you want derived class method have additional parameters and you want method call work without error on base as well. This is called “Liskov substitution principle” (or LSP) which guarantees that if person switches from base to derived instance or vice versa, they don’t have to revamp their code. To do this in Python, you need to design your base class with the following technique:

class Base:
    # simply allow additional args in base class
    def hello(self, name, *args, **kwargs):
        print("Hello", name)

class Derived(Base):
      # derived class also has unused optional args so people can
      # derive new class from this class as well while maintaining LSP
      def hello(self, name, age=None, *args, **kwargs):
          super(Derived, self).hello(name, age, *args, **kwargs) 
          print('Your age is ', age)

b = Base()
d = Derived()

b.hello('Alice')        # works on base, without additional params
b.hello('Bob', age=24)  # works on base, with additional params
d.hello('Rick')         # works on derived, without additional params
d.hello('John', age=30) # works on derived, with additional params

Above will print: 

    Hello Alice
    Hello Bob
    Hello Rick
    Your age is  None
    Hello John
    Your age is  30

.
Play with this code

More Related Contents:

  1. Inheritance of private and protected methods in Python
  2. Overriding properties in python
  3. In Python, how do I indicate I’m overriding a method?
  4. Force child class to override parent’s methods
  5. What does ‘super’ do in Python? – difference between super().__init__() and explicit superclass __init__()
  6. How do I call a parent class’s method from a child class in Python?
  7. Calling parent class __init__ with multiple inheritance, what’s the right way?
  8. Python: Bind an Unbound Method?
  9. What is the difference between a function, an unbound method and a bound method?
  10. Why does Python code use len() function instead of a length method?
  11. Why do some functions have underscores “__” before and after the function name?
  12. Cast base class to derived class python (or more pythonic way of extending classes)
  13. Subclassing int in Python
  14. Chain-calling parent initialisers in python [duplicate]
  15. Why aren’t superclass __init__ methods automatically invoked?
  16. Python super() raises TypeError
  17. Overriding “+=” in Python? (__iadd__() method)
  18. How to create a subclass in python that is inherited from turtle Module
  19. Why doesn’t 2.__add__(3) work in Python?
  20. How do I access instance variables in Python?
  21. Is it possible to make abstract classes?
  22. “TypeError: method() takes 1 positional argument but 2 were given” but I did only pass one
  23. “TypeError: method() takes 1 positional argument but 2 were given” but I only passed one
  24. Why does numpy have a corresponding function for many ndarray methods?
  25. Python calling method in class
  26. How to find instance of a bound method in Python?
  27. Python super() arguments: why not super(obj)?
  28. Why don’t my subclass instances contain the attributes from the base class (causing an AttributeError when I try to use them)?
  29. Using a class’ __new__ method as a Factory: __init__ gets called twice
  30. Python Unbound Method TypeError

Leave a Comment