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@classmethod def total_accounts(cls) -> int: return cls._total_accounts acc = BankAccount("Elena", 1000) acc.deposit(500) print(acc.balance) # 1500 print(BankAccount.total_accounts()) # 1 5. Error Handling – Fail Gracefully try/except/else/finally – the else runs only if no exception occurred.

def deposit(self, amount: float) -> None: if amount <= 0: raise ValueError("Deposit must be positive") self._balance += amount in python code

class BankAccount: """A simple bank account.""" _total_accounts = 0 # class variable def __init__(self, owner: str, balance: float = 0.0): self.owner = owner self._balance = balance BankAccount._total_accounts += 1 @classmethod def total_accounts(cls) -> int: return cls

@property def balance(self): """Read-only balance.""" return self._balance # Ugly (C-style loop) i = 0 while

def safe_divide(a, b): try: result = a / b except ZeroDivisionError: print("Cannot divide by zero") return None except TypeError: print("Please provide numbers") return None else: print("Division successful") return result finally: print("Execution finished") # always runs print(safe_divide(10, 2)) # 5.0 Always use context managers for resources.

# Ugly (C-style loop) i = 0 while i < len(items): print(items[i]) i += 1 for item in items: print(item) 2. Core Structures Expressed in Python Code Lists & List Comprehensions # Creating and transforming numbers = [1, 2, 3, 4] squares = [n**2 for n in numbers] # [1, 4, 9, 16] evens = [n for n in numbers if n % 2 == 0] # [2, 4] Dictionaries & Dict Comprehensions users = "alice": 25, "bob": 30 ages_plus_10 = name: age+10 for name, age in users.items() # 'alice': 35, 'bob': 40 Sets & Set Operations A = 1, 2, 3 B = 3, 4, 5 print(A | B) # union: 1,2,3,4,5 print(A & B) # intersection: 3 3. Functions – The Heart of Python Code Define behavior cleanly with type hints (Python 3.5+):