Debugging Failed Tests

Sometimes tests fail.

That is normal.

A failing test is useful because it shows something needs attention.

Your job is to read the failure, find the cause, and fix the code.

Why Failed Tests Matter

Failed tests help you:

• catch bugs early
• prevent broken features
• improve code quality
• fix problems faster

Example Failure

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

Test:

import unittest

class TestMath(unittest.TestCase):
    def test_add(self):
        self.assertEqual(add(2, 3), 5)

Example Result

FAIL: test_add
AssertionError: -1 != 5

How to Read It

The message means:

• actual result = -1
• expected result = 5

So the function logic is wrong.

Fix the Code

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

Common Reasons Tests Fail

1. Wrong Logic

return a - b

instead of:

return a + b

2. Wrong Expected Value

Your test may be incorrect.

3. Typing Mistakes

Wrong variable names or spelling.

4. Edge Cases Missed

The code works for normal values but fails for zero, empty input, or negatives.

Use print() for Debugging

result = add(2, 3)
print(result)

See what the function returns.

Use VS Code Debugger

Set breakpoints and inspect variables line by line.

Test One Problem at a Time

Fix the first failing test, then run tests again.

Example: String Bug

def greet(name):
    return "Hi" + name

Test expects:

Hi Tom

But actual output becomes:

HiTom

Fix:

return "Hi " + name

Code Along

Create a broken double(n) function.

Write a test.

Debug and fix it.

Mini Challenge

This code is wrong:

def square(n):
    return n * 2

Test:

self.assertEqual(square(4), 16)

Tasks:

• Find the problem
• Fix the function

Expected output after fix:

.
OK

Real World Use Case

Developers debug failing tests in websites, APIs, billing systems, and data pipelines every day.

Quiz

1. Why is a failing test useful?
2. What does AssertionError compare?
3. Name two common causes of failed tests.
4. Why fix one failure at a time?

Assignment

Write a wrong function on purpose, create a test, then debug and fix it.

Summary

You learned how to read failing tests, find bugs, and fix code with confidence.