Chapter 9: Errors & Exceptions

Objectives

• Distinguish errors vs exceptions in Python.
• Use try/except to catch exceptions.
• Leverage else and finally clauses.
• Raise built-in exceptions with raise.
• Define and use custom exception classes.
• Employ assert for sanity checks.

1. Errors vs Exceptions

An error is a problem in your code (syntax, indentation) detected before execution. An exception is a runtime event that can be caught and handled.

# SyntaxError: missing parenthesis
print "Hello"

# ZeroDivisionError at runtime
print(1/0)

2. The try/except Block

Wrap code that may fail in try, catch specific exceptions in except.

try:
    result = 10 / int(input("Enter divisor: "))
    print("Result:", result)
except ValueError:
    print("Please enter a valid integer.")
except ZeroDivisionError:
    print("Cannot divide by zero.")

3. else & finally

else runs if no exception occurs; finally always runs.

try:
    f = open("data.txt")
    data = f.read()
except FileNotFoundError:
    print("Missing file.")
else:
    print("Read data:", data[:50])
finally:
    f.close()
    print("File closed.")

4. Raising Exceptions

Use raise to trigger an exception manually.

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

# call
check_age(-5)  # ValueError: Age cannot be negative

5. Custom Exception Classes

Subclass Exception to define your own.

class InvalidConfigurationError(Exception):
    """Raised when configuration is invalid."""
    pass

def load_config(cfg):
    if "host" not in cfg:
        raise InvalidConfigurationError("Missing host setting")

You can catch it like any built-in exception:

try:
    load_config({})
except InvalidConfigurationError as e:
    print("Config error:", e)

6. Assertions

Use assert for internal sanity checks. They can be disabled with -O.

def factorial(n):
    assert n >= 0, "n must be non-negative"
    return 1 if n in (0,1) else n * factorial(n-1)

factorial(-1)  # AssertionError: n must be non-negative

Exercises

1. Write a function that reads an integer and retries until a valid int is entered.
2. Create a custom exception AuthenticationError and use it in a login function.
3. Implement file copy with try/finally to ensure files are closed.
4. Add assert statements to validate function inputs for a calculator module.