How do I create and use classes in Python?

· Category: Python Programming

Short answer

Classes in Python are defined with the class keyword. The __init__ method initializes instance attributes, and all instance methods take self as their first parameter to refer to the current object.

Steps

  1. Define the class and __init__ method.
  2. Add methods that operate on instance data.
  3. Instantiate the class by calling it like a function.
class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def bark(self):
        return f"{self.name} says woof!"

    def birthday(self):
        self.age += 1

buddy = Dog("Buddy", 3)
print(buddy.bark())
buddy.birthday()
print(buddy.age)

Tips

  • Use docstrings to document classes and methods.
  • Keep classes focused; follow the Single Responsibility Principle.
  • Instance variables are typically defined in __init__, but Python allows adding attributes dynamically.
  • Class variables are shared across all instances unless overridden at the instance level.

Common issues

  • Forgetting self in method definitions causes TypeError when calling the method.
  • Shadowing built-in names (like str or list) with attribute names can cause subtle bugs.
  • Modifying mutable class variables (like lists) affects all instances that have not overridden them.