Python Exceptions

In contrast, an exception is one among many different errors that occur while a program is being executed. The moment an exception occurs, Python will stop executing the program immediately and raise the error message. However, exceptions can be made to be handled in such a way that programs either carry on or give feedback to users.

Key Concepts

1. What Is an Exception?

An exception is an event that deviates from the normal program flow. It usually results from:

• Invalid user input
• Code that tries to divide by zero
• Files or directories missing
• Issues with internet connections or APIs
• Bugs in the code

2. Popular Exceptions in Python

Some of the most common exceptions include:

• ZeroDivisionError: This occurs when there is an attempt to divide by zero.
• IndexError: This occurs when a list is accessed with an invalid index.
• KeyError: Occurs when a key does not exist in a dictionary.
• ValueError: When a function takes an argument of correct type but incorrect value
• TypeError: The error that occurs when some operation or function is used with an object of incorrect type
• FileNotFoundError: The error that occurs when the system is unable to find a file or a directory.

Handling Exceptions

To handle exceptions, Python provides the try-except block.

Syntax:

try:
   # Code that may raise an exception
except SomeException:
   # Code to handle the exception

Example:

try:
   number = int(input("Enter a number: "))
   print(10 / number)
except ZeroDivisionError:
   print("You can't divide by zero!")
except ValueError:
   print("That's not a valid number!")

Else and Finally

• else block: Executes if no exception occurs.
• finally block: Executes no matter what, even if an exception occurs or not.

Example:

try:
   file = open("example.txt", "r")
   content = file.read()
   print(content)
except FileNotFoundError:
   print("File not found.")
else:
   print("File read successfully.")
finally:
file.close() # This will always execute

Raising Exceptions

You can raise exceptions explicitly using the raise keyword.

Example:

age = int(input("Enter your age: "))
if age < 0:
    raise ValueError("Age cannot be negative!")

Custom Exceptions

You can create your own exception classes by inheriting from the Exception class.

Example:

class NegativeNumberError(Exception):
   pass

try:
   number = int(input("Enter a positive number: "))
   if number < 0:
       raise NegativeNumberError("Negative numbers are not allowed!")
except NegativeNumberError as e:
   print(e)

Best Practices in Exception Handling

1. Handle specific exceptions: Except block should be avoided unless unavoidable.
2. Use finally for cleanup: Ensure that resources, such as files or connections, are closed.
3. Do not hide exceptions: Throw them meaningfully or let them propagate, if they can’t be resolved.
4. Log exceptions: For debugging and monitoring purposes.