B2.1.3 Describe how programs use common exception handling techniques

Exception handling prevents programs from crashing when errors occur by catching them and responding gracefully.

try:
    x = int(input("Enter a number: "))
    result = 10 / x
except ValueError:
    print("Invalid input — not a number")
except ZeroDivisionError:
    print("Cannot divide by zero")
else:
    print(f"Result: {result}")
finally:
    print("Done")

Block	Purpose
`try`	Code that may raise an exception
`except`	Handles specific exception types
`else`	Runs only if no exception was raised
`finally`	Always runs — cleanup code

Common Built-in Exceptions

Exception	Cause
`ValueError`	Wrong data type (e.g., `int("abc")`)
`ZeroDivisionError`	Division/modulo by zero
`IndexError`	List index out of range
`KeyError`	Dictionary key not found
`FileNotFoundError`	File does not exist

Raising custom exceptions

Use raise ExceptionType("message") to trigger exceptions manually.

age = -1
if age < 0:
    raise ValueError("Age cannot be negative")

Catching Any Exception

except Exception as e catches any exception and stores it in variable e, allowing you to inspect or log the error message:

try:
    x = int(input("Enter a number: "))
except Exception as e:
    print(f"Error occurred: {e}")

Advantages of Exception Handling

Advantage	Explanation
Graceful termination	Program exits cleanly via `finally` block instead of crashing abruptly — cleanup code (e.g., closing files, releasing resources) always runs
User-friendly error messages	Replaces cryptic tracebacks with plain-language feedback like `"Please enter a valid number"`
Separation of concerns	Error-handling logic is separated from normal program logic — code stays clean and readable
Selective handling	Different `except` blocks handle specific errors differently, allowing precise and appropriate responses
Prevents cascading failures	Catching an exception early stops it from propagating and causing unrelated failures elsewhere
Debugging and logging	`except Exception as e` captures the full error message and stack trace for developers to diagnose issues
Resource safety	`finally` ensures files, database connections, and network sockets are closed even when errors occur