How to Achieve Inheritance & Polymorphism in Python

The aim of this page📝 is to describe how Python’s polymorphism is achieved with the use of super() method.

• Let’s have a child/derive class — just a definition

class RefrigeratedShippingContainer(ShippingContainer):
      <       DERIVED_CLASS       ><    BASE_CLASS   >

• The statement means that class methods defined in the base get inherited in the derived
• The derived class is also known as the subclass/child class

Repeat that class attributes are defined without prefixes and called with <class_name> prefix

• Remember Constants are uppercase < PEP 8: The Style Guide for Python Code
• Also, see How to Implement Class Attributes in Python Classes
• In short, define with no prefix → call within the class with the class prefix → for example:

# DEFINE
class RefrigeratedShippingContainer(ShippingContainer):

    MAX_CELSIUS = 4.0
# CALL 
    RefrigeratedShippingContainer.MAX_CELSIUS

The overriding of __init()__ in the derived class must be explicitly done with super().__init()__

• let’s assume that we need to override the initializer of the base class because we need a new instance attribute
• in this example, it is the temperature of the container (celsius) — it occurs only in derived classes
• of course, I want to make some validation here and raise an error if the temperature is too high
• the base class has nothing to do with this and should know nothing about that → polymorphism
• we are still calling the __init__() within the base class as it sets some instance attributes we need, such as owner and the actual contents of a container, attributes that are shared among all containers
• allegedly, in other languages, it is the case that constructors at every level in an inheritance hierarchy will be called automatically

…but not in python

• in other words, the following will not call the base class __init__() ...no matter that you are defining inheritance relationship in the

RefrigeratedShippingContainer(ShippingContainer)
                              ^^^^^^^^^^^^^^^^^signature of the  class definition

• the following is incomplete and not calling __init__() from the parent (ShippingContainer)

class RefrigeratedShippingContainer(ShippingContainer):

    MAX_CELSIUS = 4.0

    def __init__(self, celsius):
        if celsius > RefrigeratedShippingContainer.MAX_CELSIUS:
            raise ValueError("Temperature too hot!")
        self.celsius = celsius

""" TYPE ERROR - YOU ARE NOT BEING EXPLICIT ABOUT YOUR PARENTS! """
>>> example = RefrigeratedShippingContainer(owner_code="MRP", contents="poetry", celsius=1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() got an unexpected keyword argument 'owner_code'

• in other words, once you start defining __init__() in a derived class, you need to be explicit
• explicit about all of the instance attributes
• Remember the zen of python: Explicit is better than implicit and thus:

1. call the built-in super() that gets the reference to the base class passed as an argument in the class definition
2. call __init__() on the returned reference and forward the constructor args
3. perform the override in what follows

class RefrigeratedShippingContainer(ShippingContainer):

    MAX_CELSIUS = 4.0 

    def __init__(self, owner_code, contents, celsius):
        super().__init__(owner_code, contents) # * calling the base class version of __init__ 
        """ 🠋 OVERRIDE 🠋 """
        if celsius > RefrigeratedShippingContainer.MAX_CELSIUS: 
            raise ValueError("Temperature too hot!")
        self.celsius = celsius

• in this case, we are inheriting the constructor from the base class with its setting of owner code + contents + with the generation of a bic_code which has the shared logic for any container anywhere

Not calling super() in the derived initializer will fire AttributeError

• if you have

class RefrigeratedShippingContainer(ShippingContainer):

    MAX_CELSIUS = 4.0

    def __init__(self, owner_code, contents, celsius):
        # super().__init__(owner_code, contents) 
        # ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ commented out
        if celsius > RefrigeratedShippingContainer.MAX_CELSIUS:
            raise ValueError("Temperature too hot!")
        self.celsius = celsius

… the instance initialization will not fail on you

• you will get an error only when you attempt to call that property that you may expect has been set (inherited!) — but it was not!
• it is not enough if the class definition refers to the inherited with

>>> f = RefrigeratedShippingContainer(owner_code="MRP",contents=["books"], celsius=-1)

# WORKS --> SET IN THE CHILD CLASS

>>> f.celsius
-1

# FAILS --> PYTHON INHERITS EXPLICITLY!
>>> f.owner_code
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'RefrigeratedShippingContainer' object has no attribute 'owner_code'

'RefrigeratedShippingContainer' object has no attribute 'owner_code'

>>> f.contents
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'RefrigeratedShippingContainer' object has no attribute 'contents'

'RefrigeratedShippingContainer' object has no attribute 'contents'