Exception Handling* in Python

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• Python Exception Handling run-time error : division by zero

Exception Handling in Python. Exception handling is crucial for making your code more robust and preventing it from crashing due to unexpected errors.

1. Try-Except Block

• Python uses the try, except block to handle exceptions.
• Code that might raise an error is placed in the try block, and the except block contains code to handle the error if it occurs.

Example:

   try:
       x = 10 / 0  # This will raise a ZeroDivisionError
   except ZeroDivisionError:
       print("Cannot divide by zero!")

2. Catching Multiple Exceptions

• You can handle different types of exceptions using multiple except blocks.
• Example:

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

3. The else Block

• The else block runs if no exceptions are raised in the try block.
• Example:

     try:
    num = int(input("Enter a number: "))
    print(f"Success! You entered {num}.")
     except ValueError:
    print("Invalid input!")
     else:
    print("No exceptions occurred.")
  

4. The finally Block

• The finally block runs no matter what, whether or not an exception was raised. It’s often used for cleanup tasks (e.g., closing files or releasing resources).

Example:

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

5. Raising Exceptions

• You can raise exceptions manually using the raise keyword. This can be useful for custom error handling.

Example:

   def check_age(age):
       if age < 18:
           raise ValueError("Age must be 18 or older.")
       else:
           print("Age is valid.")

   try:
       check_age(15)
   except ValueError as e:
       print(f"Error: {e}")

6. Creating Custom Exceptions

• You can create your own exception classes by inheriting from Python’s built-in Exception class.

Example:

   class NegativeNumberError(Exception):
       pass

   def check_number(num):
       if num < 0:
           raise NegativeNumberError("Negative numbers are not allowed.")
       else:
           print("Number is valid.")

   try:
       check_number(-5)
   except NegativeNumberError as e:
       print(f"Error: {e}")

Exercises:

• Exercise 1: Write a program that asks the user for two numbers and divides them. Handle exceptions for ValueError (non-numeric input) and ZeroDivisionError (division by zero).
• Exercise 2: Create a custom exception called TooYoungError that is raised if a user enters an age below 18. Handle this exception in the program.

Would you like to try these exercises, or would you like to explore another topic such as file handling with exceptions, or more about the try, except blocks?

Key Terms

Fix the Error in Python

7. else Block After try Without except

Code:

try:
    result = 10 / 2
else:
    print("Division successful!")

Mistake:
An else block is used incorrectly after try without an except block.

Corrected Code:

try:
    result = 10 / 2
except ZeroDivisionError:
    print("You can't divide by zero!")
else:
    print("Division successful!")

• Now, the else block runs only if no exception occurs.

True/False (Mark T for True and F for False)

Answer Key (True/False):

Multiple Choice (Select the best answer)

1. Which function would you use to determine the type of a variable in Python?
   • A) id()
   • B) type()
   • C) str()
   • D) isinstance()

What is the correct way to raise an exception in Python? a) raise Exception(“error message”) b) throw Exception(“error message”) c) exit(“error message”) d) halt(“error message”)

Watch this video for the answer:

Answer key (Mutiple Choice):

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Answer Key (Fill in the Blanks):

Exercises

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References and Bibliography

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