Python Exception Handling

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Exception Handling in Python: Complete Beginner to Advanced Guide

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Exceptions in Python are runtime errors that interrupt the normal flow of your program. If these errors go unhandled, the program immediately stops and displays an error message. To prevent such crashes and give users a smooth experience, Python provides a clean way to catch, manage, and respond to errors gracefully.

In this guide, you'll learn everything—from basic try/except usage to advanced concepts like custom exceptions, exception chaining, and handling multiple errors professionally.

Why Do We Use Exception Handling?

Without exception handling, even a small mistake—like dividing by zero or entering wrong input—can crash your application. Using exceptions allows you to:

  • Prevent your program from breaking unexpectedly
  • Show user-friendly error messages
  • Handle invalid input safely
  • Keep code clean, organized, and professional

Basic Exception Handling Syntax


try:
    # risky code
except SpecificError as e:
    # handle error
else:
    # runs only when no errors occur
finally:
    # always runs cleanup code

Example 1: Safe Division Program

Here’s a rewritten, simple, and more readable version of your earlier example:


try:
    a = float(input("Enter the numerator: "))
    b = float(input("Enter the denominator: "))
    result = a / b
    print(f"Result = {result}")
except ZeroDivisionError:
    print("Error: You cannot divide by zero.")
except ValueError:
    print("Please enter only numbers.")
finally:
    print("Calculation attempt finished.")

Possible outcomes:

Handling Multiple Exceptions

Python allows catching multiple different errors in one block:


try:
    values = [10, 5, 0]
    print(values[5])
except (IndexError, TypeError) as e:
    print(f"An error occurred: {e}")

Common Built-in Exceptions

  • ZeroDivisionError - dividing by zero
  • ValueError - invalid data conversion
  • IndexError - accessing invalid index
  • TypeError - wrong type of operand
  • FileNotFoundError - missing file

Advanced Topic 1: Creating Custom Exceptions

Custom exceptions help you build cleaner, domain-specific error handling.


class AgeError(Exception):
    pass

def validate_age(age):
    if age < 0:
        raise AgeError("Age cannot be negative.")
    return age

try:
    validate_age(-9)
except AgeError as e:
    print(e)

Advanced Topic 2: Raising & Chaining Exceptions


def secure_divide(x, y):
    try:
        return x / y
    except ZeroDivisionError as err:
        raise ValueError("Failed to divide numbers.") from err

This preserves the original error but presents a cleaner message.

Best Practices

  • Catch only specific exceptions, never use a blank except:
  • Keep your try block short
  • Always clean up resources inside finally
  • Log exception messages during debugging
  • Use custom exceptions for clarity in large projects

Summary

  • Exceptions help prevent program crashes
  • try, except, else, finally form the core error-handling structure
  • Advanced techniques like custom exceptions allow scalable design
  • Exception handling builds professional, user-friendly programs

Mastering exception handling is essential for writing robust Python applications— from small scripts to production-level systems.

Frequently Asked Questions (FAQ)

1. What is the difference between an error and an exception?

Errors are serious issues that usually cannot be recovered from, while exceptions are expected runtime issues you can handle.

2. Can I catch multiple exceptions in one block?

Yes. Python allows grouping them: except (TypeError, ValueError):

3. When should I use finally?

Use it when you must run cleanup code—like closing files or releasing connections.

4. Are custom exceptions necessary?

Not always, but they help make large applications more readable and maintainable.

5. Is catching a general Exception bad?

Yes, it can hide bugs. Only do it with logging or when absolutely required.

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