A Complete Guide - Python Programming Raising Exceptions

Step 1: Understanding Exceptions in Python

An exception is an event that disrupts the normal flow of the program. Python provides several built-in exception classes, such as ValueError, TypeError, and RuntimeError.

Step 2: The try Block

Begin by enclosing the code that might raise an exception within a try block.

try:
    x = int(input("Enter an integer: "))
except ValueError:
    print("That's not an integer!")

In this example, the input provided by the user is expected to be an integer. If the user enters something else (e.g., a string), Python raises a ValueError, which is then caught and handled in the except block.

Step 3: The raise Statement

The raise statement allows you to manually trigger an exception when a certain condition is met. Here’s how you can use it:

try:
    age = int(input("Enter your age: "))
    if age < 0:
        raise ValueError("Age cannot be negative")
except ValueError as e:
    print(f"ValueError: {e}")

In this example, we check if the age entered by the user is negative. If so, we raise a ValueError with a custom error message.

Step 4: Custom Exceptions

You can define your own exception class, which should be derived from the Exception class. This is useful for creating more specific error messages or handling unique error states.

# Define a custom exception
class NegativeAgeError(Exception):
    def __init__(self, message="Age cannot be negative"):
        self.message = message
        super().__init__(self.message)

try:
    age = int(input("Enter your age: "))
    if age < 0:
        raise NegativeAgeError()
except NegativeAgeError as e:
    print(f"NegativeAgeError: {e}")
except ValueError as e:
    print(f"ValueError: {e}")

In this example, we create a custom exception called NegativeAgeError. When a negative age is detected, this custom exception is raised and caught in the except block.

Step 5: else Block in Exception Handling

An else block can be used alongside try and except blocks. Code inside an else block runs only if no exceptions were raised in the corresponding try block.

try:
    x = int(input("Enter a number: "))
except ValueError as e:
    print(f"ValueError: {e}")
else:
    print(f"You entered the number {x}")

In this example, the else block will execute only if the conversion to an integer was successful without raising an exception.

Step 6: finally Block in Exception Handling

A finally block executes after all other blocks have been executed. It typically includes clean-up actions that must be performed under all circumstances (e.g., closing files).

try:
    file = open('example.txt', 'w')
    file.write('Hello, world!')
except IOError as e:
    print(f"IOError: {e}")
finally:
    file.close()
    print("File has been closed.")

Even if an IOError occurs while writing to the file, the finally block ensures that the file is closed properly.

Step 7: Combining Multiple Exception Types

You can handle multiple exceptions using a single except block to catch different exception types.

try:
    numerator = int(input("Enter the numerator: "))
    denominator = int(input("Enter the denominator: "))
    result = numerator / denominator
except (ZeroDivisionError, ValueError) as e:
    print(f"Error: {e}")
else:
    print(f"The result is {result}")

This example handles both ZeroDivisionError (division by zero) and ValueError (non-integer inputs).

Step 8: Using try, except, else, and finally Together

Here is a comprehensive example combining all four blocks.

def calculate_average(scores):
    try:
        total = sum(scores)
        count = len(scores)
        average = total / count
    except TypeError:
        raise TypeError("All scores must be integers or floats")
    except ZeroDivisionError:
        raise ZeroDivisionError("No scores provided")
    else:
        return average
    finally:
        print("Average calculation attempted.")

# Example usage
try:
    scores = [85, 90, 78]
    avg = calculate_average(scores)
    print(f"The average score is {avg}")
except Exception as e:
    print(e)

In this example:

• The calculate_average function computes the average of a list of scores.
• If any score in the list is not a number, a TypeError is raised.
• If the list is empty, a ZeroDivisionError is raised.
• If no exceptions occur, the else block returns the average.
• Regardless of the outcome, the finally block prints a message indicating that the calculation was attempted.

Conclusion