Exception Handling

This lesson covers exception handling for the OCR A-Level Computer Science (H446) specification. Exception handling allows programs to detect and respond to runtime errors gracefully, rather than crashing unexpectedly.

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What is an Exception?

An exception is a runtime event that disrupts the normal flow of program execution. Exceptions occur when something goes wrong during execution -- for example, dividing by zero, accessing a missing file, or providing invalid input.

Term	Definition
Exception	A runtime error that disrupts normal program flow.
Throwing/raising	Creating an exception to signal an error.
Catching/handling	Detecting an exception and executing recovery code.
Stack trace	The sequence of function calls that led to the exception.

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Common Exception Types

Exception (Python)	Cause
ValueError	Invalid value (e.g., int("hello"))
TypeError	Wrong data type (e.g., "text" + 5)
ZeroDivisionError	Division by zero
IndexError	List index out of range
KeyError	Dictionary key not found
FileNotFoundError	File does not exist
PermissionError	Insufficient permissions
AttributeError	Object does not have the attribute/method
NameError	Variable not defined
OverflowError	Numeric result too large

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Try / Catch / Finally

The try/catch/finally structure (called try/except/finally in Python) is the primary mechanism for handling exceptions.

Basic Structure

try:
    # Code that might raise an exception
    result = 10 / 0
except ZeroDivisionError:
    # Code that runs if the specific exception occurs
    print("Error: Cannot divide by zero")
finally:
    # Code that ALWAYS runs, whether or not an exception occurred
    print("This always executes")

Catching Multiple Exception Types

try:
    number = int(input("Enter a number: "))
    result = 100 / number
    print(f"Result: {result}")
except ValueError:
    print("Error: Please enter a valid integer")
except ZeroDivisionError:
    print("Error: Cannot divide by zero")
except Exception as e:
    print(f"Unexpected error: {e}")

The else Clause

The else block runs only if no exception was raised:

try:
    number = int(input("Enter a number: "))
except ValueError:
    print("Invalid input")
else:
    print(f"You entered: {number}")  # Only runs if no exception
finally:
    print("Done")  # Always runs

Execution Flow

try block
    |
    +-- No exception --> else block --> finally block
    |
    +-- Exception raised --> matching except block --> finally block

Exam Tip: The finally block always executes, even if there is a return statement in the try or except block. This makes it ideal for cleanup operations like closing files or database connections.

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Pseudocode (OCR Style)

TRY
    number = USERINPUT
    number = INT(number)
    result = 100 / number
    OUTPUT result
EXCEPT ValueError
    OUTPUT "Please enter a valid number"
EXCEPT ZeroDivisionError
    OUTPUT "Cannot divide by zero"
ENDTRY

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Throwing (Raising) Exceptions

You can raise exceptions deliberately to signal that something has gone wrong.

def set_age(age: int):
    if age < 0:
        raise ValueError("Age cannot be negative")
    if age > 150:
        raise ValueError("Age cannot exceed 150")
    return age

try:
    set_age(-5)
except ValueError as e:
    print(f"Error: {e}")  # Error: Age cannot be negative

When to Raise Exceptions

• When input validation fails.
• When a precondition for a function is not met.
• When an unexpected state is detected.
• When a resource is unavailable.

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Custom Exceptions

You can create your own exception classes by inheriting from the built-in Exception class:

class InsufficientFundsError(Exception):
    def __init__(self, balance: float, amount: float):
        self.balance = balance
        self.amount = amount
        super().__init__(
            f"Cannot withdraw {amount}: only {balance} available"
        )

class BankAccount:
    def __init__(self, balance: float):
        self.__balance = balance