Exception Handling

Exception Handling

What it means

Exceptions are Python’s mechanism for signaling and handling errors at runtime. Code that might fail goes in a try block; except catches specific exception types and handles them; finally runs regardless of whether an exception occurred (cleanup code); else runs only if no exception was raised.

Examples

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Can't divide by zero")
except (TypeError, ValueError) as e:
    print(f"Bad input: {e}")
else:
    print("No errors occurred")   # only runs if try succeeded
finally:
    print("This always runs")    # cleanup, e.g. closing a file

# Raising your own exception
def withdraw(balance, amount):
    if amount > balance:
        raise ValueError("Insufficient funds")
    return balance - amount

Common mistake

Using a bare except: (catching everything) — this silently swallows real bugs, including things like KeyboardInterrupt, making problems much harder to diagnose.

# Hides everything, including typos and bugs
try:
    process(data)
except:
    pass

# Catches only what you expect, and re-raises unknowns visibly
try:
    process(data)
except ValueError as e:
    log.warning(f"Skipping bad record: {e}")

Quick practice

1. What’s wrong with a bare except: clause?

   Answer

   It catches every exception, including ones you didn't anticipate and system-level ones like KeyboardInterrupt, hiding real bugs instead of surfacing them.

2. When does the finally block run?

   Answer

   Always — whether or not an exception was raised, and even if the try or except block returns or re-raises.

3. What’s the difference between else and finally in a try statement?

   Answer

   else runs only if no exception occurred in try; finally runs unconditionally, exception or not.