Robert Heaton

Software Engineer /
One-track lover / Down a two-way lane

PFAB#6: Real-world debugging practice

28 Dec 2019

This post is part of my “Programming Feedback for Advanced Beginners” series, which helps you make the leap from knowing syntax to writing clean, elegant code. Subscribe now to receive PFAB in your inbox, every weekend, entirely free.

Only half of programming is about writing code. The other half is about figuring out why the code you’ve written doesn’t god damn work.

This post contains 3 excerpts from real-world programs that my readers have sent me, asking for assistance figuring out why their code is broken. Your task is to find the bugs and fix them. These bugsquashes are great practice for thinking methodically, and for understanding code that you didn’t originally write. You don’t need to do any setup in order to tackle them, and you can even run the code directly in your browser.

Before you start, here’s a step-by-step, whirlwind guide to what I like to call “scientific debugging”:

A very short guide to scientific debugging

For me, un-scientific debugging typically means alternating between:

  • Sitting perfectly still, staring at my screen and waiting for divine inspiration to strike
  • Flailing around trying semi-random changes that are very rarely actually summoned from the heavens

By contrast, scientific debugging means forming explicit hypotheses about what is wrong with my code (which may themselves very well be wrong), systematically trying to prove or disprove them, and then repeating this process, taking me inexorably closer and closer to a cure.

Here’s how you can debug scientifically too:

  1. Start by writing a short snippet of test code that precisely verifies whether your code is behaving correctly or not. This makes it easy to test your bugfixes (in the example bugs below I’ve added some test snippets for you).
  2. If possible, form a hypothesis for what you think might be the problem (“I wonder if I’m forgetting to subtract 1 from the length of my array”). If you’re feeling especially rigorous, write this hypothesis down and take notes throughout this entire process.
  3. If you are able to form a hypothesis, work out how to prove or disprove it. For example “if this hypothesis is correct then the code should work for inputs below 0, but break for those above 0” or “if I add a print statement here then this variable should be None”.
  4. If you are able to prove that your hypothesis is correct, use this new knowledge to fix your code. Re-run your test case from step 1. If it passes, verify that your code works for other test cases too. This ensures that you haven’t introduced new bugs or missed some even subtler edge-cases.
  5. On the other hand, if your investigations instead dis-prove your hypothesis, this is still great progress. Now you understand your code better and know something that isn’t wrong. Try to use your new knowledge to form a new hypothesis. Loop this process forever until you find the answer or give up and ask a friend.

If at any point you aren’t able to come up with a new hypothesis (“what do you think is wrong?” “I literally have no idea”), add print-statements all over your code in order to better understand what is going on inside it. Form micro-hypotheses about how you think the internals of your code are working (“I believe that at this point in the code first_name should be Robert). Prove or disprove these hypotheses (“huh, it’s actually obert”). Use this new knowledge to form hypotheses about the larger problems you are seeing (“ohhh I bet I’m accidentally chopping off the first character of every input, which would explain why I can’t find my users in my database”). Prove or disprove them and repeat as above.

We’ll talk in much more detail about scientific debugging in future PFABs, including examples and common patterns. Here’s something I wrote about it a few years ago. But for now, here are today’s bugs.

The bugs

The bugs are written in Python, but the concepts in them are entirely generic. The bugs are all logic mistakes that you could easily accidentally write in almost any language. You don’t need to know anything about esoteric Python syntax in order to tackle the bugs, other that in order to print the variable x to the terminal you write print(x).

If you fix the bugs, get stuck, or have any other questions or suggestions, please do let me know. I’d love to hear from you.

1. Blank board

This bug was sent to me by someone working on Programming Projects for Advanced Beginners #3: Tic-Tac-Toe. They were trying to write a function that would generate and return a blank Tic-Tac-Toe board, but something was going wrong. See if you can figure out what.

To view, run, and edit this code in your browser click here.

def blank_board(width, height):
    This function should return a "2-D array" that
    we can use to represent a 2-D board for a game
    like Tic-Tac-Toe or Battleships. For more on
    using 2-D arrays, see:

    board = []
    row = []
    for _ in range(height):
        for _ in range(width):
        return board

if __name__ == "__main__":
    test_board = blank_board(4, 3)

    expected_board = [
        [None, None, None, None],
        [None, None, None, None],
        [None, None, None, None],
    # TODO: it looks like there's a bug with our
    # blank-board generating function! We should
    # figure out what it is and fix it so that
    # this test passes.
    if test_board == expected_board:
        print("TEST PASSED!!")
        print("TEST FAILED!!")


2. Search Results

This bug was sent to me by someone who was trying to programatically query an online search engine with multiple search terms, and then combine all the results into a single array. But things were not going well. Why not?

To view, run, and edit this code in your browser click here.

def search_all(queries):
    all_results = []
    for q in queries:
        results = search(q)
    return all_results

def search(query):
    This function is a "fake" search engine
    that we use to make this bugsquashing
    exercise more concise. In the original
    program this function queried an
    internet search engine.

    Now it returns "fake" search results
    by appending "-0", "-1", and "-2" to
    the given query. For example, the query
    "banana" will return:

    ["banana-0", "banana-1", "banana-2"]

    Note that using "fake" functions to
    simplify programs while testing or
    debugging is not cheating - it is a very
    common and sensible technique!
    results = []
    for i in range(3):
        results.append(query + "-" + str(i))
    return results

if __name__ == "__main__":
    test_queries = ["cat", "dog", "mouse"]
    test_results = search_all(test_queries)

    expected_results = [
    # TODO: it looks like there's a bug with
    # our searching function! We should
    # figure out what it is and fix it so
    # that this test passes.
    if test_results == expected_results:
        print("TEST PASSED!!")
        print("TEST FAILED!!")


3. Power Battles

Finally, this bug was sent to me by someone building a command-line adventure game. Things were going well, but their players were losing battles that they shouldn’t have been losing. Find out why and save the day.

To view, run, and edit this code in your browser click here.


def player_has_enough_power_to_defeat_enemy(player_stats, enemy_stats):
    # Subtract the enemy power from the
    # player power and make sure that
    # the difference is more than 5.
    player_stats['power'] = player_stats['power'] - enemy_stats['power']

    if player_stats['power'] > EXTRA_POWER_NEEDED_TO_DEFEAT_ENEMY:
        return True
        return False

if __name__ == "__main__":
    # In our game, the player defeats an
    # enemy if they have more than
    # 5 greater power than the enemy.
    player_stats = {
        'name': 'Horatio',
        'power': 35,

    enemy1_stats = {
        'name': 'Death Face',
        'power': 27,
    enemy2_stats = {
        'name': 'Brain Eater',
        'power': 20,

    # TODO: since our player has more than
    # 5 extra power than both Death Face
    # and Brain Eater, we expect them to
    # defeat both enemies. However, this
    # is not happening! We should figure
    # out what the bug is and fix it.
    if player_has_enough_power_to_defeat_enemy(player_stats, enemy1_stats):
        print("Player DEFEATS Enemy 1!")
        print("Player LOSES to Enemy 1!")

    if player_has_enough_power_to_defeat_enemy(player_stats, enemy2_stats):
        print("Player DEFEATS Enemy 2!")
        print("Player LOSES to Enemy 2!")


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