Exception Handling

This lesson covers exception handling — the mechanism by which programs detect, report, and recover from runtime errors gracefully. Exception handling is essential for building robust software that does not crash unexpectedly when something goes wrong.

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What is an Exception?

An exception is an event that occurs during program execution that disrupts the normal flow of instructions. Exceptions are typically caused by:

Cause	Example
Invalid input	A user enters "abc" when a number is expected.
Division by zero	Attempting to divide by zero.
File not found	Trying to open a file that does not exist.
Index out of range	Accessing an array element beyond its bounds.
Type errors	Performing an operation on an incompatible data type.
Network errors	A connection to a server fails.

Without exception handling, any of these errors would cause the program to crash with an error message.

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Try-Except (Try-Catch) Blocks

Exception handling uses a structured approach with try, except (or catch in other languages), and optionally finally blocks.

Pseudocode

TRY
    x = INPUT("Enter a number: ")
    x = INT(x)
    result = 100 / x
    OUTPUT "Result: " + STR(result)
EXCEPT ZeroDivisionError
    OUTPUT "Error: Cannot divide by zero."
EXCEPT ValueError
    OUTPUT "Error: Please enter a valid integer."
FINALLY
    OUTPUT "Program complete."
END TRY

Python

try:
    x = int(input("Enter a number: "))
    result = 100 / x
    print(f"Result: {result}")
except ZeroDivisionError:
    print("Error: Cannot divide by zero.")
except ValueError:
    print("Error: Please enter a valid integer.")
finally:
    print("Program complete.")

How It Works

1. The code inside the try block is executed.
2. If an exception occurs, execution jumps to the matching except block.
3. If no exception occurs, the except blocks are skipped.
4. The finally block (if present) always executes, whether or not an exception occurred.

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Common Exception Types

Exception Type	When It Occurs
ValueError	A function receives an argument of the right type but wrong value (e.g., int("abc")).
TypeError	An operation is applied to an object of the wrong type (e.g., "hello" + 5).
ZeroDivisionError	Division or modulo by zero.
IndexError	An index is outside the range of a list or array.
KeyError	A dictionary key is not found.
FileNotFoundError	A file or directory is requested but cannot be found.
IOError	An I/O operation fails (e.g., reading from a corrupted file).
OverflowError	A numerical result is too large to be represented.
RecursionError	Maximum recursion depth exceeded.

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Multiple Except Blocks

You can handle different exception types with separate except blocks, allowing tailored responses to different errors.

def safe_divide(a, b):
    try:
        result = a / b
        return result
    except ZeroDivisionError:
        print("Cannot divide by zero")
        return None
    except TypeError:
        print("Both arguments must be numbers")
        return None

Catching All Exceptions

try:
    # risky code
    pass
except Exception as e:
    print(f"An error occurred: {e}")

Exam Tip: Catching all exceptions with a bare except or except Exception is generally considered bad practice because it can mask bugs. Always catch specific exceptions where possible. However, you should know that it can be used as a "safety net" in production code.

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The finally Block