Python Overriding Method

Summary: in this tutorial, you’ll learn how to use Python overriding method to allow a child class to provide a specific implementation of a method that is provided by one of its parent classes.

Introduction to Python overridding method

The overriding method allows a child class to provide a specific implementation of a method that is already provided by one of its parent classes.

Let’s take an example to understand the overriding method better.

First, define the Employee class:

class Employee: def __init__(self, name, base_pay): self.name = name self.base_pay = base_pay def get_pay(self): return self.base_pay
Code language: Python (python)

The Employee class has two instance variables name and base_pay. It also has the get_pay() method that returns the base_pay.

Second, define the SalesEmployee that inherits from the Employee class:

class SalesEmployee(Employee): def __init__(self, name, base_pay, sales_incentive): self.name = name self.base_pay = base_pay self.sales_incentive = sales_incentive
Code language: Python (python)

The SalesEmployee class has three instance attributes: name, base_pay, and sales_incentive.

Third, create a new instance of the SalesEmployee class and display the pay:

john = SalesEmployee('John', 5000, 1500) print(john.get_pay())
Code language: Python (python)

Output:

5000
Code language: Python (python)

The get_pay() method returns only the base_pay, not the sum of the base_pay and sales_incentive.

When you call the get_pay() from the instance of the SalesEmployee class, Python executes the get_pay() method of the Employee class, which returns the base_pay.

To include the sales incentive in the pay, you need to redefine the get_pay() method in the SalesEmployee class as follows:

class SalesEmployee(Employee): def __init__(self, name, base_pay, sales_incentive): self.name = name self.base_pay = base_pay self.sales_incentive = sales_incentive def get_pay(self): return self.base_pay + self.sales_incentive
Code language: Python (python)

In this case, we say that the get_pay() method in the SalesEmployee class overrides the get_pay() method in the Employee class.

When you call the get_pay() method of the SalesEmployee‘s object, Python will call the get_pay() method in the SalesEmployee class:

john = SalesEmployee('John', 5000, 1500) print(john.get_pay())
Code language: Python (python)

Output:

6500
Code language: Python (python)

If you create an instance of the Employee class, Python will call the get_pay() method of the Employee class, not the get_pay() method of the SalesEmployee class. For example:

jane = Employee('Jane', 5000) print(jane.get_pay())
Code language: Python (python)

Put it all together.

class Employee: def __init__(self, name, base_pay): self.name = name self.base_pay = base_pay def get_pay(self): return self.base_pay class SalesEmployee(Employee): def __init__(self, name, base_pay, sales_incentive): self.name = name self.base_pay = base_pay self.sales_incentive = sales_incentive def get_pay(self): return self.base_pay + self.sales_incentive if __name__ == '__main__': john = SalesEmployee('John', 5000, 1500) print(john.get_pay()) jane = Employee('Jane', 5000) print(jane.get_pay())
Code language: Python (python)

Advanced method overriding example

The following defines the Parser class:

class Parser: def __init__(self, text): self.text = text def email(self): match = re.search(r'[a-z0-9\.\-+_]+@[a-z0-9\.\-+_]+\.[a-z]+', self.text) if match: return match.group(0) return None def phone(self): match = re.search(r'\d{3}-\d{3}-\d{4}', self.text) if match: return match.group(0) return None def parse(self): return { 'email': self.email(), 'phone': self.phone() }
Code language: Python (python)

The Parser class has an attribute text which specifies a piece of text to be parsed. Also, the Parser class has three methods:

  • The email() method parses a text and returns the email.
  • The phone() method parses a text and returns a phone number in the format nnn-nnnn-nnnn where n is a number from 0 to 9 e.g., 408-205-5663.
  • The parse() method returns a dictionary that contains two elements email and phone. It calls the email() and phone() method to extract the email and phone from the text attribute.

The following uses the Parser class to extract email and phone:

s = 'Contact us via 408-205-5663 or email@test.com' parser = Parser(s) print(parser.parse())
Code language: Python (python)

Output:

{'email': 'email@test.com', 'phone': '408-205-5663'}
Code language: Python (python)

Suppose you need to extract phone numbers in the format n-nnn-nnn-nnnn, which is the UK phone number format. Also, you want to use extract email like the Parser class

To do it, you can define a new class called UkParser that inherits from the Parser class. In the UkParser class, you override the phone() method as follows:

class UkParser(Parser): def phone(self): match = re.search(r'(\+\d{1}-\d{3}-\d{3}-\d{4})', self.text) if match: return match.group(0) return None
Code language: Python (python)

The following use the UkParser class to extract a phone number (in UK format) and email from a text:

s2 = 'Contact me via +1-650-453-3456 or email@test.co.uk' parser = UkParser(s2) print(parser.parse())
Code language: Python (python)

Output:

{'email': 'email@test.co.uk', 'phone': '+1-650-453-3456'}
Code language: Python (python)

In this example, the parser calls the parse() method from the parent class which is the Parser class. In turn, the parse() method calls the email() and phone() methods.

However, the parser() doesn’t call the phone() method of the Parser class but the phone() method of the UkParser class:

parser.parse()
Code language: Python (python)

The reason is that inside the parse() method, the self is the parser which is an instance of the UkParser class.

Therefore, when you call self.phone() method inside the parse() method, Python will look for the phone() method that is bound to the instance of the UkParser.

Put it all together.

import re class Parser: def __init__(self, text): self.text = text def email(self): match = re.search(r'[a-z0-9\.\-+_]+@[a-z0-9\.\-+_]+\.[a-z]+', self.text) if match: return match.group(0) return None def phone(self): match = re.search(r'\d{3}-\d{3}-\d{4}', self.text) if match: return match.group(0) return None def parse(self): return { 'email': self.email(), 'phone': self.phone() } class UkParser(Parser): def phone(self): match = re.search(r'(\+\d{1}-\d{3}-\d{3}-\d{4})', self.text) if match: return match.group(0) return None if __name__ == '__main__': s = 'Contact us via 408-205-5663 or email@test.com' parser = Parser(s) print(parser.parse()) s2 = 'Contact me via +1-650-453-3456 or email@test.co.uk' parser = UkParser(s2) print(parser.parse())
Code language: Python (python)

Overriding attributes

The following shows how to implement the Parser and UkParser classes by overriding attributes:

import re class Parser: phone_pattern = r'\d{3}-\d{3}-\d{4}' def __init__(self, text): self.text = text def email(self): match = re.search(r'[a-z0-9\.\-+_]+@[a-z0-9\.\-+_]+\.[a-z]+', self.text) if match: return match.group(0) return None def phone(self): match = re.search(self.phone_pattern, self.text) if match: return match.group(0) return None def parse(self): return { 'email': self.email(), 'phone': self.phone() } class UkParser(Parser): phone_pattern = r'(\+\d{1}-\d{3}-\d{3}-\d{4})' if __name__ == '__main__': s = 'Contact us via 408-205-5663 or email@test.com' parser = Parser(s) print(parser.parse()) s2 = 'Contact me via +1-650-453-3456 or email@test.co.uk' parser = UkParser(s2) print(parser.parse())
Code language: Python (python)

In this example, the Parser has a class variable phone_pattern. The phone() method in the Parser class uses the phone_pattern to extract a phone number.

The UkParser child class redefines (or overrides) the phone_pattern class attribute.

If you call the parse() method from the UkParser‘s instance, the parse() method calls the phone() method that uses the phone_pattern defined in the UkParser class.

Summary

  • Method overrding allows a child class to provide a specific implementation of a method that is already provided by one of its parent class.
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