Stop 06 · Programming Fundamentals for AutomationTopics · 8/10

Debugging Basics

In short

Your test failed. Is the app broken or is your script broken? Debugging is how you find out calmly, instead of changing random lines until it turns green.

Systematically finding why a script isn't working as expected — using print statements, breakpoints, or a debugger, rather than randomly changing code and hoping it works.

The purple blob sits on a mat checking a string of small fairy light bulbs with a handheld tester.

Think of it like this

One fused bulb in the fairy lights

The whole string of party lights is dark. You don't throw it away. You test from one end, bulb by bulb, or check the middle first to halve the search. Debugging a script is the same patient hunt.

Watch it happen

A calm way to debug

Reproduce

Read error

Isolate

Inspect

Fix, rerun

1/5Run the failing test alone. Does it fail every time or only sometimes?

The big ideas

What you need to know

Errors are clues

TimeoutException, KeyError, AssertionError each point somewhere different. Read before you react.

Breakpoints pause time

breakpoint() in Python or a red dot in VS Code stops the script so you can look around.

Print still works

A quick print of a variable often answers the question faster than guessing.

App bug or test bug?

Try the same steps by hand. If it works manually, suspect your script first.

See it in code

Walk through the code

python

1def cart_total(prices, discount_percent):2    subtotal = sum(prices)3    breakpoint()  # pause here and inspect subtotal4    return subtotal - subtotal * discount_percent / 1005 6 7total = cart_total([499, 299], 10)8assert round(total, 2) == 718.2, f"Expected 718.2, got {total}"9print("Cart total check passed")

1/3Execution stops here. You can print variables and step line by line.

breakpoint() drops you into the pdb debugger. Type p subtotal to print a value, c to continue.

Save yourself the pain

Common mistakes

Oops

Changing three things at once, then not knowing which one fixed it.

Do this instead

Change one thing, rerun, observe. Slow is fast here.

Oops

Only reading the first line of a long stack trace.

Do this instead

Scroll to the bottom. The last frames usually point at your own file and line.

Your mission

Debug on purpose

~25 min

Quick check

A UI test fails, but you do the same steps by hand and everything works. What's the most likely next step?

Go deeper

1Practice using a debugger (or strategic print statements) to trace through a failing script.
2Learn to isolate a problem to its smallest reproducible case before trying to fix it.
3Build the habit of reading the full error message before assuming what went wrong.