Inheritance

Describing Inheritance

• Inheritance refers to creating a new class that modifies the behavior of an existing class

• In this case:

  • The original class is called a base class or superclass
  • The new class is called a derived class or subclass
• The subclass inherits the attributes defined by its base class
• The following is an example of a inheritance:

>>> class EvilAccount(Account):
...     def inquiry(self):
...         return 'bad inquiry'

>>> a = EvilAccount('Todd', 200)
>>> print(a.inquiry())
'bad inquiry'

• In this example, instances of EvilAccount and Account are identical except for the redefined instance() method

Implementing Inheritance in Python

• Inheritance is implemented with only a slight change to the dot operator

• Specifically, the search for an attribute does the following:

  1. First, tries to find a match in the instance
  2. Next, tries to find a match in the instance's class
  3. Then, tries to move on to the base class
  4. Then, tries to continue moving up to previous base classes

The super Object in Python

• The super function returns a proxy object that allows us to access methods of a base class

• This function has two uses:

  • Avoids using the base class name explicitly
  • Simplifies working with multiple inheritance
• Specifically, an attribute is searched using the normal search rules that would have been used on the base classes
• This frees us from hard-coding the exact location of a method

References

• Python Essential Reference