April Python Bootcamp Day 15

 

---

What is Exception Handling?

Exception handling is the process of responding to runtime errors so that the normal flow of the program is not interrupted.

Example without Exception Handling

num = int(input("Enter a num: "))
print(10 / num)
print("Hello World")

If the user enters 0 or invalid input, the program crashes and "Hello World" will not execute.

---

try - except Block

To prevent crashes, Python provides the try-except mechanism.

• try → Code that may cause an error
• except → Code that handles the error

Basic Example

try:
    num = int(input("Enter a num: "))
    print(10 / num)
except:
    print("Something went wrong!")

print("Hello World")

Now, even if an error occurs, the program continues execution.

---

Handling Specific Exceptions

Handling specific exceptions is always better than using a general except.

Example

try:
    num = int(input("Enter a num: "))
    print(10 / num)
except ZeroDivisionError:
    print("Cannot divide by zero")
except ValueError:
    print("Invalid input! Please enter a number")
except:
    print("Unaware of the error")

print("Hello World")

This improves debugging and makes your code more precise.

---

else and finally

Python provides two additional blocks:

• else → Runs when no exception occurs
• finally → Always runs

Example

try:
    file = open("data.txt", "r")
    print(file.read())
except FileNotFoundError as e:
    print("File Not Found", e)
else:
    print("Found the file")
finally:
    print("Execution completed")

---

Multiple Exceptions in One Block

You can handle multiple exceptions together:

try:
    num = int(input("Enter a num: "))
    print(10 / num)
except (ValueError, ZeroDivisionError):
    print("Something went wrong!")

---

Using Exception Objects

You can capture the exception details using as.

try:
    x = int("abc")
except ValueError as e:
    print("Error:", e)

---

Raising Exceptions Manually

You can trigger exceptions using the raise keyword.

age = int(input("Enter age: "))

if age < 18:
    raise ValueError("You must be 18 or older")

print("Access Granted")

---

Custom Exceptions

You can define your own exception classes by inheriting from Exception.

Example

class MyError(Exception):
    pass

raise MyError("This is a custom error")

---

Real-World Example: Bank Withdrawal System

class InsufficientBalanceError(Exception):
    pass

balance = 5000
withdraw = int(input("Enter amount to withdraw"))

try:
    if withdraw > balance:
        raise InsufficientBalanceError("Not enough balance")
    else:
        print("Withdrawal successful")
except InsufficientBalanceError as e:
    print(e)

This demonstrates how custom exceptions can model real-world scenarios.

---

Best Practices

• Always handle specific exceptions instead of generic ones
• Use finally for cleanup tasks (closing files, releasing resources)
• Avoid silent failures (empty except)
• Use custom exceptions for domain-specific logic

---

Assignment Questions

Basic Level

1. Write a program that takes a number as input and handles invalid input using try-except.
2. Create a program that divides two numbers and handles division by zero.
3. Demonstrate the use of else in exception handling.

---

Intermediate Level

4. Write a program to open a file and handle the case when the file does not exist.
5. Handle multiple exceptions (ValueError, ZeroDivisionError) in a single block.
6. Capture and print exception details using as.

---

Advanced Level

7. Create a custom exception called NegativeNumberError and raise it when a negative number is entered.
8. Build a login system that raises an exception if the password is incorrect.
9. Modify the bank withdrawal system to:
   • Allow multiple transactions
   • Update balance after withdrawal
   • Handle invalid inputs

---

Challenge Question

10. Create a menu-driven program that:

• Takes user input
• Performs operations (division, file reading, etc.)
• Uses proper exception handling for all cases