Exception Handling Study Notes for PCAP Certification Exam

Exception handling is a vital concept for PCAP (Certified Associate in Python Programming) certification exam. It is a mechanism that allows you to handle and recover from errors that may occur during the execution of your code.

In this study guide, we will cover the fundamental concepts of exception handling in Python, along with examples to help you prepare for the PCAP certification exam.

What is Exception Handling in Python?
Exception handling in Python is the process of identifying, handling, and recovering from errors that occur during the execution of a program. It helps in preventing the program from crashing when unexpected errors occur and also assists in identifying and debugging these errors.

Python Try...Except Block
The try...except block is the primary method of handling exceptions in Python. Its syntax is as follows:

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

In this block, the code that may cause an exception is placed in the try block, while the except block catches and handles the exception that occurs. It is crucial to note that every try block must be followed by an except block.

Example: Handling a ZeroDivisionError

try:
    numerator = 10
    denominator = 0

    result = numerator/denominator

    print(result)
except:
    print("Error: Denominator cannot be 0.")

# Output: Error: Denominator cannot be 0. 

In this example, we divide a number by 0, which would result in a ZeroDivisionError. By placing the code inside the try block, we can catch the error in the except block and handle it accordingly.

Catching Specific Exceptions in Python
In addition to handling any exception that may occur, the except block can also be used to catch specific exceptions. This allows you to handle different types of errors in different ways. Here's an example:

try:
    even_numbers = [2,4,6,8]
    print(even_numbers[5])

except ZeroDivisionError:
    print("Denominator cannot be 0.")

except IndexError:
    print("Index Out of Bound.")

# Output: Index Out of Bound 

In this code, we attempt to access an element at index 5, which does not exist, and this results in an IndexError. The second except block catches and handles this specific error. If no error occurs in the try block, the except blocks are skipped.

Python Try with Else Clause
Sometimes, you may want to execute a specific block of code only if the try block runs without any errors. You can achieve this by adding an optional else keyword after the try block. Let's see an example:

# program to print the reciprocal of even numbers

try:
    num = int(input("Enter a number: "))
    assert num % 2 == 0  #  The assert statement checks that num is even.
except:
    print("Not an even number!")
else:
    reciprocal = 1/num
    print(reciprocal)

Run Code
Output:

If we enter an odd number:

Enter a number: 1
Not an even number!

If we enter an even number, the reciprocal is computed and displayed.

Enter a number: 4
0.25

However, if we enter 0, we get a ZeroDivisionError as the code inside the else block is not handled by the preceding except blocks.

Enter a number: 0
Traceback (most recent call last):
File "", line 7, in
reciprocal = 1/num
ZeroDivisionError: division by zero

Here, the assert statement in the code checks if num is an even number. If it is not, an AssertionError is raised, which is then caught by the except block.

Note: Exceptions in the else block are not caught by the preceding except blocks.

Python Try...Finally
The finally block in Python is always executed, whether an exception occurs or not. It is useful for handling cleanup code, such as closing files or database connections, regardless of whether an error occurs in the code or not. Here's an example:

try:
    numerator = 10
    denominator = 0

    result = numerator/denominator

    print(result)
except:
    print("Error: Denominator cannot be 0.")

finally:
    print("This is the finally block.")

Run Code
Output:

Error: Denominator cannot be 0.
This is the finally block.

In this example, the code inside the finally block will always be executed, regardless of whether an error occurs or not.

Conclusion
Exception handling is a crucial skill for anyone preparing for the PCAP certification exam. In this study guide, we covered the basics of exception handling in Python, including the try...except block, catching specific exceptions, using the else clause, and the finally block. Understanding and mastering these concepts will help you write robust and error-free Python applications, making you well-prepared for the PCAP certification exam.