Writing Simple Tests

Before using test libraries, it is important to understand the idea of a test.

A simple test checks whether a function returns the correct result.

What Is a Test Case?

A test case includes:

• input
• expected output
• actual result

Example:

• Input: add(2, 3)
• Expected: 5

Example Function

def add(a, b):
    return a + b

Manual Check

print(add(2, 3))

If the output is 5, the test passes.

Better: Compare Results

def add(a, b):
    return a + b

result = add(2, 3)

print(result == 5)

Output

True

Why This Helps

Instead of only printing values, you directly check if the answer is correct.

Multiple Tests

def add(a, b):
    return a + b

print(add(2, 3) == 5)
print(add(1, 1) == 2)
print(add(0, 5) == 5)

Output

True
True
True

Example with Failure

def subtract(a, b):
    return a + b

print(subtract(5, 2) == 3)

Output

False

The test found a bug.

Create Pass / Fail Messages

def add(a, b):
    return a + b

if add(2, 3) == 5:
    print("PASS")
else:
    print("FAIL")

Output

PASS

Why Test Different Inputs?

A function may work for one case and fail for another.

Test:

• normal values
• zero
• negative values
• edge cases

Code Along

Create function:

def square(n):
    return n * n

Write three tests.

Mini Challenge

Create function:

def is_even(n):
    return n % 2 == 0

Write tests for:

• 2
• 3
• 0

Expected output:

True
True
True

(Each line means the test passed.)

Real World Use Case

Developers write tests for totals, forms, APIs, reports, and calculations.

Quiz

1. What is a test case?
2. Why compare actual and expected results?
3. What does False mean in a test?
4. Why test multiple inputs?

Assignment

Create a triple(n) function and write four tests for it.

Summary

You learned how to write simple tests by comparing expected and actual results.