Python 24-Day Course - Day 12: Exception Handling

Day 12: Exception Handling

Basic Exception Handling

try:
    number = int(input("Enter a number: "))
    result = 100 / number
    print(f"Result: {result}")
except ValueError:
    print("Please enter a valid number")
except ZeroDivisionError:
    print("Cannot divide by zero")

try-except-else-finally

try:
    f = open("data.txt", "r")
    content = f.read()
except FileNotFoundError:
    print("File not found")
else:
    print(f"File content: {content}")  # Runs only if no error
finally:
    print("Operation complete")  # Always runs

Common Built-in Exceptions

Exception	When It Occurs
ValueError	Invalid value conversion
TypeError	Invalid type operation
KeyError	Dictionary key not found
IndexError	Index out of range
FileNotFoundError	File not found
ZeroDivisionError	Division by zero
AttributeError	Accessing non-existent attribute

Custom Exceptions

class InsufficientBalanceError(Exception):
    def __init__(self, balance, amount):
        self.balance = balance
        self.amount = amount
        super().__init__(
            f"Insufficient balance: balance {balance}, withdrawal request {amount}"
        )

def withdraw(balance, amount):
    if amount > balance:
        raise InsufficientBalanceError(balance, amount)
    return balance - amount

try:
    result = withdraw(1000, 5000)
except InsufficientBalanceError as e:
    print(e)  # Insufficient balance: balance 1000, withdrawal request 5000

Today’s Exercises

1. Create a function that validates user input to accept only integers between 1 and 100.
2. Write a safe file reading function that handles all possible exceptions.
3. Create a NegativeNumberError custom exception and use it in practice.