Debugging with Tests

When a bug is reported, most developers’ first instinct is to open the app and try to find it manually. A more professional approach is to use Tests as a Debugging Tool.

The “Red-Fix-Green” Debugging Workflow

1. Reproduce with a Test

Before you touch your source code, write a test that fails exactly how the bug was described. This is your “reproduction case.”

# The Bug: The discount calculator is giving 100% off instead of 10%
def test_discount_bug():
    # This test fails currently, proving the bug exists
    assert calculate_discount(100, 0.10) == 10 

2. Isolate the Problem

Because you have a small, repeatable test, you can now use a debugger or print statements to see exactly what’s happening inside that function without having to restart the whole app.

3. Apply the Fix

Modify your code until the test turns Green.

4. Regression Prevention

Now that the test passes, keep it in your codebase. This ensures that no one else (including you) accidentally reintroduces this bug six months from now.

Why this is better than “Print Debugging”

• Repeatability: You don’t have to manually re-type the same inputs into a form 50 times.
• Speed: Running a single test takes a fraction of a second.
• Documentation: The test documents the “edge case” that caused the bug for future developers.

Summary

A bug is just a test case you haven’t written yet. Once you write it, the bug becomes a permanent part of your quality suite.