Python Exception Handling

What is an Exception?

An exception is an error that occurs at runtime — Python encounters something it can't handle and stops the program. Without exception handling, a single bad input crashes everything. The try/except block lets you catch specific errors and respond gracefully, keeping the program running and giving users helpful messages.

try / except

Code that might fail goes inside try. If an exception occurs, Python jumps to the matching except block and runs it. If no exception occurs, except is skipped. Name the exception as e to access its message.

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")

try:
    num = int("hello")
except ValueError as e:
    print(f"Bad conversion: {e}")

Cannot divide by zero!
Bad conversion: invalid literal for int() with base 10: 'hello'

Multiple except + finally

You can have multiple except clauses for different error types. The finally block always runs — whether or not an exception occurred. Use it for cleanup that must always happen (closing connections, releasing resources).

def safe_div(a, b):
    try:
        print(f"Result: {a / b}")
    except ZeroDivisionError:
        print("Zero division!")
    except TypeError:
        print("Wrong types!")
    finally:
        print("--- done ---")

safe_div(10, 2)
safe_div(10, 0)

Result: 5.0
--- done ---
Zero division!
--- done ---

Raising Exceptions

Use raise to intentionally trigger an exception when invalid data is detected. This enforces rules in your code — for example, rejecting a negative age or a too-short password.

def set_age(age):
    if age < 0 or age > 150:
        raise ValueError(f"Age {age} is out of range (0–150)")
    return age

try:
    set_age(-5)
except ValueError as e:
    print(f"Error: {e}")

Error: Age -5 is out of range (0–150)