Exception Handling in Python

What You’ll Learn in This Blog

• Why exception handling is important
• Common runtime errors in Python
• Using try, except, else, finally blocks
• Catching specific vs. generic exceptions
• Raising exceptions manually with raise
• Creating your own custom exceptions
• Best practices with real-world examples

Why Do We Need Exception Handling?

In read-world applications, errors will happen — a user might provide wrong input, a file might be missing, or an API might fail.

Instead of crashing the program, Python provides a graceful way to handle errors using exception handling.

Common Runtime Errors (Exceptions)

Here are a few examples:

Basic Structure of Exception Handling

try:
	# risky code
except SomeError:
	# handle the error
else:
	# runs if no exception
finally:
	# always runs

Example: Try and Except

try:
	num = int(input("Enter a number: "))
	result = 10 / num
	print("Result:", result)
except ZeroDivisionError:
	print("You cannot divide by zero.")
except ValueError:
	print("Please ener a valid number.")

Using else and finally

• else runs only if no error occurs
• finally always runs (cleanup code, closing files, etc.)

try:
	f = open("data.txt", "r')
	content = f.read()
except FileNotFoundError:
	print("File not found!")
else:
	print("File read successfully.")
finally:
	f.close()
	print("File Closed.")

Catching Multiple Exceptions Together

try:
	value = int("abc")
	print(10 / 0)
except (ValueError, ZeroDivisionError) as e:
	print("Caught an error:", e)

Raising Exceptions Manually with raise

You can raise exceptions if needed:

age = -5
if age < 0:
	raise ValueError("Age cannot be negative!")

Custom Exceptions in Python

Create your own exceptions by inheriting from Exception:

class InvalidAgeError(Exception):
	pass


age = int(input("Enter age: "))
if age < 0:
	raise InvalidAgeError("Custom: Age can't be negative.")

Best Practices

• Be specific with exception types (avoid except: alone)
• Log errors for debugging (use logging module)
• Clean up using finally
• Use custom exceptions to make errors meaningful
• Avoid hiding bugs by catching too broadly

Practice Task

Create a function that:

• Accepts a list of numbers.
• Returns the average.
• Raises and handles;
  • TypeError if input is not a list
  • ZeroDivisionError if list is empty

Summary Table