Python Classroom notes 11/Jul/2026

Design Patterns

• In programming languages we have two terms
  • Patterns:
    • Solutions to known problems
    • This gives you the right way of implementation for an existing problem which is already solved by someone
  • AntiPatterns
    • Never follow this
• Design Patterns: Refer Here
• Design Patterns are classified into
  • Creational Patterns
  • Structural Patterns
  • Behavioral Patterns

Problem

• I have a class which will get the current us dollar to indian rupee conversion
• This class has a method convert which converts the currency according to current market
• This is used in many places in my application.
• Every where we use we create an object and then call convert
• If an object needs to be created only once we use singleton pattern

Solution – Singleton

Python object creation process

• Non clean singleton

class CurrencyConvertor:
    instance = None

    def __new__(cls):
        print("1. __new__ called")
        if cls.instance is not None:
            return CurrencyConvertor.instance
        cls.instance = super().__new__(cls)
        return cls.instance

    def __init__(self):
        print("inside init")

    def convert(self, value, source_currency, destination_currency):
        pass


if __name__ == '__main__':
    c1 = CurrencyConvertor()
    c2 = CurrencyConvertor()
    print(c1 == c2)