Lesson 9 of 40
OOP
Intermediate
⏱ 35 min
Classes & OOP Fundamentals
Build models with Python classes — instance and class variables, __init__, properties, dunder methods, and Pythonic object design.
Part 1: Defining a Class
class BankAccount:
"""A simple bank account."""
interest_rate = 0.05 # class variable
def __init__(self, owner: str, balance: float = 0.0):
self.owner = owner # instance variable
self._balance = balance # _ = "private by convention"
@property
def balance(self) -> float:
return self._balance
def deposit(self, amount: float):
if amount <= 0: raise ValueError("Positive amount required")
self._balance += amount
"""A simple bank account."""
interest_rate = 0.05 # class variable
def __init__(self, owner: str, balance: float = 0.0):
self.owner = owner # instance variable
self._balance = balance # _ = "private by convention"
@property
def balance(self) -> float:
return self._balance
def deposit(self, amount: float):
if amount <= 0: raise ValueError("Positive amount required")
self._balance += amount
Part 2: Dunder (Magic) Methods
class Vector:
def __init__(self, x:float, y:float):
self.x, self.y = x, y
def __repr__(self): return f"Vector({self.x}, {self.y})"
def __add__(self, other): return Vector(self.x+other.x, self.y+other.y)
def __len__(self): return int((self.x**2+self.y**2)**0.5)
def __eq__(self, other): return (self.x,self.y) == (other.x,other.y)
def __init__(self, x:float, y:float):
self.x, self.y = x, y
def __repr__(self): return f"Vector({self.x}, {self.y})"
def __add__(self, other): return Vector(self.x+other.x, self.y+other.y)
def __len__(self): return int((self.x**2+self.y**2)**0.5)
def __eq__(self, other): return (self.x,self.y) == (other.x,other.y)
Part 3: @classmethod & @staticmethod
class User:
def __init__(self, name, email):
self.name, self.email = name, email
@classmethod
def from_dict(cls, data: dict) -> "User":
return cls(data["name"], data["email"])
@staticmethod
def validate_email(email: str) -> bool:
return "@" in email
def __init__(self, name, email):
self.name, self.email = name, email
@classmethod
def from_dict(cls, data: dict) -> "User":
return cls(data["name"], data["email"])
@staticmethod
def validate_email(email: str) -> bool:
return "@" in email
Part 4: __slots__ for Memory Efficiency
class Point:
__slots__ = ("x", "y") # no __dict__ — saves ~50% memory
def __init__(self, x:float, y:float):
self.x, self.y = x, y
# Creating millions of points is now much faster
points = [Point(i, i**2) for i in range(1_000_000)]
__slots__ = ("x", "y") # no __dict__ — saves ~50% memory
def __init__(self, x:float, y:float):
self.x, self.y = x, y
# Creating millions of points is now much faster
points = [Point(i, i**2) for i in range(1_000_000)]