Exception Handling (try/except)

Exception Handling

Introduction

Exception handling allows your program to respond to errors gracefully instead of crashing. Python uses try/except blocks to catch and handle exceptions.

Exception Flow: try → except → else → finally

flowchart TB
    Try["🔷 try:
Code that might fail"] --> Exception{"❌ Exception
raised?"}

    Exception -->|"Yes"| Except["🔶 except:
Handle the error"]
    Exception -->|"No"| Else["✅ else:
Runs if NO exception
(success path)"]

    Except --> Finally["✅ finally:
ALWAYS runs
(cleanup: close files,
release resources)"]
    Else --> Finally
    Try --> Finally

    style Try fill:#4f46e5,color:#fff
    style Exception fill:#f59e0b,color:#000
    style Except fill:#dc2626,color:#fff
    style Else fill:#059669,color:#fff
    style Finally fill:#7c3aed,color:#fff

Basic Syntax

try:
    # Code that might raise an exception
    risky_operation()
except SomeException:
    # Code to handle the exception
    handle_error()

try/except

try:
    number = int(input("Enter a number: "))
    print(f"100 / {number} = {100 / number}")
except ValueError:
    print("That's not a valid number!")
except ZeroDivisionError:
    print("Cannot divide by zero!")

Catching Multiple Exceptions

try:
    value = int(input("Enter a number: "))
    result = 100 / value
    print(result)
except (ValueError, ZeroDivisionError) as e:
    print(f"Error: {e}")

else and finally

try:
    file = open("data.txt", "r")
    content = file.read()
except FileNotFoundError:
    print("File not found!")
else:
    # Runs only if no exception occurred
    print(f"Read {len(content)} characters")
finally:
    # ALWAYS runs — great for cleanup
    if 'file' in locals():
        file.close()
        print("File closed")

Raising Exceptions

def withdraw(balance, amount):
    if amount < 0:
        raise ValueError("Amount cannot be negative")
    if amount > balance:
        raise InsufficientFundsError("Insufficient balance")
    return balance - amount

Best Practices

Be specific — catch specific exceptions, not bare except:
Don’t swallow exceptions — unless you handle them meaningfully
Use finally for cleanup — close files, release resources
Prefer else — keep success logic separate from error handling
Raise early, catch late — validate inputs at boundaries

Practice Exercises

Exercise 1: Write a function that safely parses a list of strings into integers, returning only the valid ones.

Exercise 2: Create a simple bank account class that raises custom exceptions for invalid operations.