12 - Exception Handling

An exception is a runtime event that stops normal execution unless the program handles it.

A simple picture

An exception is like an emergency card passed from one helper to another. Normal work pauses. A helper that knows this exact problem may handle it and continue safely; otherwise the card keeps moving outward.

risky operation -> exception -> matching except block -> recovery

Types of problems

  • A syntax error means Python cannot understand the program.
  • A runtime exception occurs while understandable code is running.
  • A logic error runs successfully but produces the wrong result.

Examples of runtime exceptions:

int("ten")       # ValueError
10 / 0           # ZeroDivisionError
open("missing")  # FileNotFoundError

Basic try and except 

try:
    age = int(input("Age: "))
except ValueError:
    print("Please enter a whole number.")
else:
    print(f"Age recorded: {age}")

Put only the risky operation in the try block. Catch the specific exception you expect.

Important fact: try does not mean “ignore every problem.” It marks an operation that may fail in an expected way. Catch only exceptions you understand and can handle correctly.

Multiple exceptions 

try:
    numerator = float(input("Numerator: "))
    denominator = float(input("Denominator: "))
    print(numerator / denominator)
except ValueError:
    print("Use numeric values.")
except ZeroDivisionError:
    print("The denominator cannot be zero.")

Different errors deserve different recovery messages.

else and finally

else runs only when no exception occurred. finally runs whether the operation succeeded or failed:

file = None
try:
    file = open("notes.txt", encoding="utf-8")
    print(file.read())
except FileNotFoundError:
    print("Notes file does not exist.")
finally:
    if file is not None:
        file.close()

For files, prefer with open(...); it handles cleanup automatically.

Raising an exception

Use raise when a function receives data that violates its contract:

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

Raising an exception is not the same as handling it. The caller can decide how to present the problem.

Validation loop 

while True:
    try:
        number = int(input("Enter a number: "))
        break
    except ValueError:
        print("That was not a whole number. Try again.")

The loop repeats only for the expected invalid-input case.

Common mistakes

  • Catching every error with bare except.
  • Putting the entire program inside one huge try block.
  • Showing a technical traceback to a beginner user when recovery is possible.
  • Silently ignoring an exception with pass.
  • Using exceptions to hide a logic error.

Bug Hunter

Bug 1 — wrong exception type 

try:
    age = int("ten")
except ZeroDivisionError:
    print("Please enter a whole number")

Bug 2 — risky operation outside try 

number = int(input("Number: "))
try:
    print(number)
except ValueError:
    print("Please enter a whole number")

Bug 3 — error silently hidden 

try:
    total = 100 / 0
except Exception:
    pass
Show Bug Hunter fixes 
# Bugs 1 and 2
try:
    age = int(input("Age: "))
except ValueError:
    print("Please enter a whole number")

# Bug 3
try:
    total = 100 / 0
except ZeroDivisionError:
    print("The denominator cannot be zero")
Optional deeper look: how does an exception find a handler?

When an exception is raised, Python stops the current normal path and looks for a matching handler. If the current function has none, Python finishes that call frame and checks its caller. This process is called stack unwinding. finally blocks and context-manager cleanup still run during unwinding.

Practice

Try these problems on this page. For each one, name the operation that can fail and the friendly message the user should see.

Problems

  1. Safely convert user input into an integer.
  2. Handle division by zero.
  3. Handle a missing file.
  4. Reject a negative age with raise.
  5. Keep asking until the user enters a valid number.
  6. Predict which exception each short program raises.
  7. Make a calculator continue after invalid operations.
  8. Validate a menu choice and report a useful message.
  9. Use else and finally correctly in a file-reading program.
  10. Build a robust command-line expense entry program.
Show hints
  1. Catch ValueError around int().
  2. Catch ZeroDivisionError or check the divisor first.
  3. Catch FileNotFoundError.
  4. Check the value and raise ValueError with a clear message.
  5. Put input inside a loop and leave only after success.
  6. Read the failing operation and match it to an exception type.
  7. Catch expected errors inside the loop.
  8. Check membership in the allowed choices.
  9. else is for success; finally runs every time.
  10. Validate each field before saving it.
Show solution ideas
  1. Put int(text) inside try and handle ValueError.
  2. Handle ZeroDivisionError with a message about the denominator.
  3. Catch the missing path and offer a recovery message.
  4. if age < 0: raise ValueError("age cannot be negative").
  5. Use while True, break after successful conversion, and a specific handler.
  6. Bad numeric text is ValueError; zero division is ZeroDivisionError; a missing path is FileNotFoundError.
  7. Keep the loop outside the individual operation attempt.
  8. Use if choice not in allowed.
  9. Put success-only code in else and cleanup in finally.
  10. Catch expected errors at the user-interface boundary and keep the program running.

Homework

Make the expense tracker reject invalid amounts, negative values, missing descriptions, and unavailable files with helpful messages.

Checkpoint

Make a calculator that continues running after invalid input and handles division by zero. Explain which exception each handler catches.