Tutorial

Python Exception Handling – Complete Try-Except Guide

Master Python exception handling with try-except-else-finally blocks. Learn best practices, custom exceptions, and error handling patterns.

📌 Python exception handling, try except, Python errors, custom exceptions, finally block, else clause

Exception handling is your reliable protector against chaos in code. It prevents crashes and makes programs more robust by gracefully managing errors that would otherwise terminate your application.

Exceptions are events that disrupt normal program execution—like dividing by zero, accessing missing files, or converting invalid strings to numbers. Python's try-except mechanism intercepts these disruptions before they crash your program.

The basic try-except structure wraps risky code in a try block and specifies exception types to catch in except blocks. This separates error handling logic from normal business logic, making code cleaner and more maintainable.

Python's complete exception handling structure includes four components: try (risky code), except (error handlers), else (success-only code), and finally (cleanup that always runs). Understanding when each executes is crucial for robust error handling.

The else clause executes only when no exception occurs—perfect for success-dependent code. The finally block always executes regardless of exceptions, making it ideal for cleanup operations like closing files or database connections.

Code Examples

Basic Try-Except

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Error: division by zero!")
    result = None

# Prevents crash, handles error gracefully

Multiple Exception Types

try:
    file_name = input("Enter file name: ")
    file = open(file_name, "r")
    line = file.readline()
    number = int(line.strip())
    result = 100 / number

except FileNotFoundError:
    print(f"File {file_name} not found")

except ValueError:
    print("Cannot convert data to a number")

except ZeroDivisionError:
    print("Error: division by zero")

Using Else and Finally

try:
    number = int("42")

except ValueError:
    print("This is not a number")

else:
    # Only runs if no exception
    print(f"Success! Number: {number}")
    print(f"Square: {number ** 2}")

finally:
    # Always runs for cleanup
    print("Operation completed")

Custom Exceptions

class InvalidEmailError(Exception):
    """Raised when an email doesn't match the format"""
    pass

def validate_email(email):
    if "@" not in email:
        raise InvalidEmailError("Email must contain @ symbol")
    print(f"Email {email} is valid")

# Usage
try:
    validate_email("invalid.email")
except InvalidEmailError as e:
    print(f"Validation failed: {e}")

Real-World File Handling

def read_config(filename):
    config = {}
    file = None

    try:
        file = open(filename, 'r')
        for line in file:
            key, value = line.strip().split('=')
            config[key] = value

    except FileNotFoundError:
        print(f"Config file {filename} not found, using defaults")
        config = {"theme": "dark", "language": "en"}

    except ValueError:
        print("Invalid config format")

    finally:
        if file:
            file.close()  # Always cleanup

    return config