Warmup

1. Write a function called first_and_last that takes a string as input.

2. The function should return a new string made up of:

   • the first character of the input string, and
   • the last character of the input string.

Examples:

first_and_last("Python")   # returns "Pn"
first_and_last("cat")      # returns "ct"
first_and_last("a")        # returns "aa"

Agenda

• Monday: Unit 4 Review
• Tuesday: Unit 4 Quiz (20 questions, Canvas)
• Wednesday: Midterm Review
• Friday: Python Midterm (40 questions, Canvas)

Lesson 4.6: Raising Exceptions

Review

• Exceptions happen when something goes wrong in a program
• try/except lets us handle those problems
• Now: how do we make our own exceptions?

The raise Keyword

• Use raise to trigger an exception on purpose

def divide(a, b):
    if b == 0:
        raise ZeroDivisionError("You can’t divide by zero!")
    return a / b

print(divide(10, 2))
print(divide(10, 0))  # raises ZeroDivisionError

Why Raise Exceptions?

• Stops bad data from breaking the program later
• Makes code more predictable
• Forces errors to be handled where appropriate

Raising Built-in Exceptions

• Common ones to raise:
  • ValueError
  • TypeError
  • ZeroDivisionError

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

Raising Custom Messages

• Add helpful error messages for debugging

x = -5
if x < 0:
    raise ValueError("x must not be negative")

Raising in Functions

• Functions can enforce rules

def square_root(n):
    if n < 0:
        raise ValueError("Cannot take square root of negative number")
    return n ** 0.5

Quick Check

• What does raise do in Python?
• Why would you raise an exception instead of just printing a message?
• Which built-in exceptions might you raise for invalid input?

Practice Challenge

Write a function get_grade(score) that:

• Raises a ValueError if the score is less than 0 or greater than 100
• Otherwise returns the letter grade (A, B, C, D, F)