Classes and Object-Oriented Programming
Python · Core Language · Run with python3
Explanation
Python uses class-based OOP with dynamic dispatch. `__init__` is the initializer (not a constructor — `__new__` allocates the object). Instance attributes are set on `self`; class attributes are shared across instances. `@classmethod` receives the class as first arg; `@staticmethod` receives neither. Properties (`@property`) give attribute-style access to computed values.
Real-World Scenario
A `BankAccount` class encapsulates balance state. The balance property enforces a read-only interface, `deposit`/`withdraw` enforce business rules (no negative balances), and `@classmethod from_dict` provides an alternate constructor for deserialising from JSON. ```python class BankAccount: interest_rate = 0.02 # class attribute def __init__(self, owner: str, balance: float = 0.0): self._owner = owner self._balance = balance @property def balance(self) -> float: return self._balance def deposit(self, amount: float) -> None: if amount <= 0: raise ValueError("Deposit must be positive") self._balance += amount def withdraw(self, amount: float) -> None: if amount > self._balance: raise ValueError("Insufficient funds") self._balance -= amount @classmethod def from_dict(cls, data: dict) -> "BankAccount": return cls(data["owner"], data.get("balance", 0.0)) def __repr__(self) -> str: return f"BankAccount(owner={self._owner!r}, balance={self._balance:.2f})" acc = BankAccount.from_dict({"owner": "Alice", "balance": 1000}) acc.deposit(500) acc.withdraw(200) print(acc) # BankAccount(owner='Alice', balance=1300.00) ```
Quick Snippet
class BankAccount:
interest_rate = 0.02 # class attribute
def __init__(self, owner: str, balance: float = 0.0):
self._owner = owner
self._balance = balance
@property
def balance(self) -> float:
return self._balance
def deposit(self, amount: float) -> None:
if amount <= 0:
raise ValueError("Deposit must be positive")
self._balance += amount
def withdraw(self, amount: float) -> None:
if amount > self._balance:
raise ValueError("Insufficient funds")
self._balance -= amount
@classmethod
def from_dict(cls, data: dict) -> "BankAccount":
return cls(data["owner"], data.get("balance", 0.0))
def __repr__(self) -> str:
return f"BankAccount(owner={self._owner!r}, balance={self._balance:.2f})"
acc = BankAccount.from_dict({"owner": "Alice", "balance": 1000})
acc.deposit(500)
acc.withdraw(200)
print(acc) # BankAccount(owner='Alice', balance=1300.00)Full Example
# Compile: python3 classes.py
# Output:
# BankAccount(owner='Alice', balance=1300.00)
class BankAccount:
interest_rate = 0.02
def __init__(self, owner: str, balance: float = 0.0):
self._owner = owner
self._balance = balance
@property
def balance(self) -> float:
return self._balance
def deposit(self, amount: float) -> None:
if amount <= 0:
raise ValueError("Deposit must be positive")
self._balance += amount
def withdraw(self, amount: float) -> None:
if amount > self._balance:
raise ValueError("Insufficient funds")
self._balance -= amount
@classmethod
def from_dict(cls, data: dict) -> "BankAccount":
return cls(data["owner"], data.get("balance", 0.0))
def __repr__(self) -> str:
return f"BankAccount(owner={self._owner!r}, balance={self._balance:.2f})"
acc = BankAccount.from_dict({"owner": "Alice", "balance": 1000})
acc.deposit(500)
acc.withdraw(200)
print(acc)
# Class vs instance attribute
a1 = BankAccount("Bob")
a2 = BankAccount("Carol")
print(BankAccount.interest_rate) # 0.02 — shared class attribute
BankAccount.interest_rate = 0.025
print(a1.interest_rate, a2.interest_rate) # both 0.025
python3 classes.py