17-exceptions

17 · Exceptions & Error Handling

Part 4 — Real-World Python · Estimated time: 25–35 min · Prerequisite: 13 · Functions

When something goes wrong at runtime — bad input, a missing file, dividing by zero — Python raises an exception. If you ignore it, your program crashes. Exception handling lets you catch the problem, respond gracefully, and keep going. This is what separates fragile scripts from robust programs.

What you'll learn

• What an exception is and what an uncaught one does
• try / except to catch errors
• Catching specific exception types
• else and finally
• raise-ing your own exceptions

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1. try / except

Wrap risky code in try. If it raises an exception, the matching except block runs instead of crashing.

try:
    number = int("abc")     # raises ValueError
    print(number)
except ValueError:
    print("That wasn't a valid number.")

print("The program keeps running.")

Without the try, int("abc") would stop the whole program.

▶️ Run it: python examples/01_try_except.py

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2. Catch specific exceptions

Catch the exact kind of problem you expect — it's clearer and safer than catching everything. You can grab the error object with as.

def divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        return "You can't divide by zero!"

print(divide(10, 2))   # 5.0
print(divide(10, 0))   # You can't divide by zero!

try:
    value = int("oops")
except (ValueError, TypeError) as e:   # catch more than one type
    print("Problem:", e)

Common built-in exceptions: ValueError, TypeError, ZeroDivisionError, KeyError, IndexError, FileNotFoundError.

▶️ Run it: python examples/02_specific_exceptions.py

💡 Try it yourself: what exception does [1, 2, 3][5] raise? Catch it and print a friendly message.

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3. else and finally

• else runs only if no exception happened.
• finally runs no matter what — perfect for cleanup.

try:
    result = 10 / 2
except ZeroDivisionError:
    print("Division failed")
else:
    print("Success:", result)   # only when there was no error
finally:
    print("This always runs")   # always, error or not

▶️ Run it: python examples/03_else_finally.py

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4. Raising your own exceptions

Use raise to signal that something is wrong — then callers can catch it.

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

try:
    set_age(-1)
except ValueError as e:
    print("Caught:", e)

▶️ Run it: python examples/04_raising.py

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Common mistakes

Mistake	What happens	Fix
Bare except: catching everything	hides real bugs (even typos)	Catch specific types: except ValueError:.
Putting too much in the try	unclear which line failed	Keep only the risky line(s) inside try.
Using exceptions for normal flow	slow, confusing	Use if checks for expected cases.
Silently pass-ing in except	errors vanish without a trace	At least log or print what happened.

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Recap / cheat-sheet

try:
    risky()                 # code that might fail
except SomeError as e:
    handle(e)               # runs if that error occurs
except (ErrA, ErrB):
    ...                     # catch several types
else:
    ...                     # runs if NO exception
finally:
    cleanup()               # always runs

raise ValueError("message") # signal an error yourself

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Exercises

Run a file with python exercises/<file>.py, then compare with the matching file in solutions/.

1. exercises/01_safe_divide.py — handle division by zero.
2. exercises/02_parse_int.py — catch a ValueError from a bad conversion.
3. exercises/03_validate_age.py — raise and catch your own error.

Try each yourself before opening the solution.

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Next → 18 · File Handling (coming soon)