Warmup

Write a function is_sum_even that takes two integers and returns whether their sum is even. Starter code with tests:

def test_func(result, expected):
    assert result == expected, f"result:{result} expected:{expected}"

# replace with your implementation of is_sum_even

cases = [
    (is_sum_even(3, 1), True),
    (is_sum_even(3, 4), False),
]

# loop over the cases and call test_func on each one
for result, expected in cases:
    test_func(result, expected)

Agenda

• Exceptions for the rest of the week
• Monday: Unit 4 practice
• Unit 4 Quiz moved to next Tuesday
  • 20 questions in Canvas, rehearse grade
• Next Wednesday: Midterm review & practice
• Thursday: midterms begin

Lesson 4.4: Errors and Exceptions

Review

• Functions let us structure programs
• Scope controls where variables live
• Now: what happens when something goes wrong?

What is an Error?

• An error happens when Python cannot complete an instruction
• Example:

print(5 / 0)   # ZeroDivisionError

• Errors stop the program unless handled

Syntax Errors vs. Exceptions

• Syntax Error: mistake in code structure (caught before running)

if True print("hi")   # SyntaxError

• Exception: happens while code is running

nums = [1, 2, 3]
print(nums[5])   # IndexError

Common Exceptions

• ZeroDivisionError – dividing by zero
• ValueError – wrong type of value

int("hello")   # ValueError

• IndexError – list index out of range
• KeyError – dictionary key doesn’t exist

Unhandled Exceptions

• If not handled, program crashes
• Example:

name = input("Enter a number: ")
number = int(name)  # crashes if input is not a number

Why Learn Exceptions?

• Real programs deal with unexpected input
• Handling exceptions keeps programs from crashing
• Builds robust software

Quick Check

• What’s the difference between a syntax error and an exception?
• Name two common exceptions.
• What happens when an exception is not handled?

Practice Challenge

Write code that tries to divide two numbers.

• First try dividing normally.
• Then, test dividing by zero and observe the error message.