Python OOP Cheat Sheet

Object-oriented Python covering classes, inheritance, dunder methods, computed properties, class/static methods, and abstract base classes.

2 PagesIntermediateMar 30, 2026

Classes & Instances

Defining a class with instance and class attributes.

python

class Dog:    species = "Canis familiaris"  # class attribute, shared    def __init__(self, name, age):        self.name = name          # instance attribute        self.age = age    def bark(self):        return f"{self.name} says Woof!"d = Dog("Rex", 3)d.bark()  # 'Rex says Woof!'

Inheritance & super()

Extending classes and calling parent implementations.

python

class Animal:    def __init__(self, name):        self.name = name    def speak(self):        raise NotImplementedErrorclass Cat(Animal):    def speak(self):        return f"{self.name} says Meow"class Kitten(Cat):    def speak(self):        return super().speak() + " (softly)"

Common Dunder Methods

Special methods that customize built-in behavior.

__init__(self, ...)- called when an instance is created, sets up attributes
__repr__(self)- unambiguous developer-facing string representation
__str__(self)- readable user-facing string, used by print() and str()
__eq__(self, other)- defines behavior for the == operator
__len__(self)- defines behavior for len(obj)
__add__(self, other)- defines behavior for the + operator
__iter__(self)- makes an object iterable with for loops
__enter__ / __exit__- implement the context manager protocol for with statements

Properties, classmethod & staticmethod

Computed attributes and alternative constructors.

python

class Circle:    def __init__(self, radius):        self._radius = radius    @property    def area(self):        return 3.14159 * self._radius ** 2    @classmethod    def unit_circle(cls):        return cls(1)          # receives the class, not an instance    @staticmethod    def is_valid_radius(r):        return r > 0            # no access to self or clsc = Circle(2)c.area              # computed on access, no parentheses needed

Abstract Base Classes

Enforcing that subclasses implement required methods.

python

from abc import ABC, abstractmethodclass Shape(ABC):    @abstractmethod    def area(self):        ...class Square(Shape):    def __init__(self, side):        self.side = side    def area(self):        return self.side ** 2# Shape()  # raises TypeError: Can't instantiate abstract class

Pro Tip

Implement `__repr__` on every class you write, even quick ones — it makes debugging and REPL sessions vastly easier, and if `__str__` is missing Python falls back to `__repr__` automatically.

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