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Python abstraction with abstract classes

Specify required behavior while leaving implementation to subclasses.

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Before you start

Inheritance and methods.

Specify required behavior while leaving implementation to subclasses.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

An abstract base class can declare methods that concrete subclasses must implement. It describes what callers may request without prescribing one implementation.

Shape requires area. Rectangle supplies it using width and height. Instantiating Shape itself would raise TypeError because its abstract method is not implemented.

Code example

Python

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        pass

class Rectangle(Shape):
    def __init__(self, width, height):
        self.width = width
        self.height = height
    def area(self):
        return self.width * self.height

shape = Rectangle(3, 4)
print(shape.area())
assert Rectangle(0, 4).area() == 0

Output

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

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1from abc import ABC, abstractmethod

3class Shape(ABC):

4 @abstractmethod

5 def area(self):

6 pass

8class Rectangle(Shape):

9 def __init__(self, width, height):

10 self.width = width

11 self.height = height

12 def area(self):

13 return self.width * self.height

15shape = Rectangle(3, 4)

16print(shape.area())

17assert Rectangle(0, 4).area() == 0

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Define the abstract contract.	`—`	—
Create a concrete instance.	`width = 3, height = 4`	—
Compute the rectangle implementation of area.	`return = 12`	—
Print the result.	`return = 12`	12
Check a zero-width rectangle.	`return = 12`	12

Behind the scenes

How Python processes this code

Python runs this source through its interpreter. It creates objects for values and resolves names as each statement runs.

Cost and common mistakes

O(1) arithmetic operations and O(1) object state.

This example assumes nonnegative numeric dimensions.
An abstract base class is useful for an explicit contract, but is not required for all Python polymorphism.

Try it yourself

Implement Square with one side and an area method.

Reveal answer and reasoning

Store side in __init__, return side * side, and check that Square(5).area() is 25.