“My code isn’t working” is a great problems-and-solutions flowchart for Python programmers

Tap to view at full size.

Dr. Martin Jones, the author behind the book and site Python for Biologists, has come up with a chart to help Python programmers when their code doesn’t work and they can’t figure out why.

He writes:

A few times a year, I have the job of teaching a bunch of people who have never written code before how to program from scratch. The nature of programming being what it is, the same error crop up every time in a very predictable pattern. I usually encourage my students to go through a step-by-step troubleshooting process when trying to fix misbehaving code, in which we go through these common errors one by one and see if they could be causing the problem. Today, I decided to finally write this troubleshooting process down and turn it into a flowchart in non-threatening colours.

Behold, the “my code isn’t working” step-by-step troubleshooting guide! Follow the arrows to find the likely cause of your problem – if the first thing you reach doesn’t work, then back up and try again.

It’s intended for programmers who are new to Python, but even experienced Pythonistas sometimes get distracted and stuck on simple things. I’m keeping a copy handy.

You can tap on the image above to view it at full size, and there’s also a printable PDF version as well.

[ Thanks to my UC Baseline classmate Daniel Jimenez for the find! ]


“I hope Ren’Py will notice me!”

What is Ren’Py?

Ren’Py is a tool for creating visual novels and an engine for running them.

There are a couple of ways to think of visual novels:

  1. As a “Choose Your Own Adventure”-style book, but in electronic form, and backed with visuals, sound effects, music, and interactivity, or
  2. As a story-driven, turn-based multimedia game, which can fit any number of genres, including adventures, simulations, or role-playing games.

Why is it called Ren’Py?

Ren’Py is a portmanteau of renai (恋愛), Japanese for “romantic love”…

…and Python, the programming language in which it’s implemented, and one of the languages you can use to create Ren’Py visual novels / games.

How much programming do I need to know to make visual novels or games in Ren’Py?

You’ve got options!

  • If you’re new to programming, Ren’py provides a scripting language that’s easy enough to let you get started writing visual novels after a couple of minutes’ worth of learning, but powerful enough to add a surprising amount of interactivity.
  • If you know Python or are an experienced programmer, you can harness the entire Python language and its libraries and geek out to your heart’s content.

And, yes, you can program using a mix of both Ren’Py’s programming language and Python.

What platforms can I develop Ren’Py visual novels and games on?

You can run the Ren’Py development tool on Windows, macOS, and Linux…

…and with a little work, you can even do Ren’Py development on a Raspberry Pi!

Aside from Ren’Py, do I need to install anything else?

You’ll need a text editor specifically made for coding — feel free to use your favorite one.

Of course, if you also want to create you own graphics, sound effects, and music for your visual novel or game, you’ll need the appropriate software for those tasks as well.

Once I’ve made a visual novel or game in Ren’Py, how can other people play it?


Ren’py games can be easily packaged for the following platforms:

  • Windows
  • macOS
  • Linux

With a little extra work, they can also be packaged for these platforms:

  • iOS
  • Android
  • Web (currently in beta)

And with even more work (and the right amount of luck), they can be deployed on these platforms:

  • Steam
  • Playstation
  • Xbox
  • Nintendo Switch

How do I get started?

You can get started by downloading the current version of Ren’py from the “Latest” page on At the time of writing, the latest version is 7.3.5, code-named “The world (wide web) is not enough”, released October 17, 2019.

Ren’Py comes with two projects:

There’s Tutorial, a Ren’Py tutorial in visual novel form, where “Professor Eileen” takes you through the basics. It’s well worth going through at least once.

There’s also The Question, which introduces you to the concept of visual novels, the interactivity, and that never-ending visual novel and anime trope, “Boy is too shy to ask the girl of his dreams on a date.”

Be sure to look at the code for both — you’ll pick up a lot just by reading it.

Watch this blog!

I’ll post some Ren’Py articles and links to the source code for a couple of games soon.


Programming What I’m Up To

Computer Coach’s “Intro to Python Coding” course (taught by Yours Truly) starts tonight!

The online Intro to Python Coding course that I’m teaching on behalf of Tampa Bay’s own Computer Coach Training Center starts tonight at 6:00 p.m.. For the next five weeks, on Monday and Wednesday evenings from 6:00 to 10:00, I’ll be leading a class of Python learners through “code along with me” exercises in the Python programming language.

Photo: Joey deVilla points at a projected screen of code with co-presented Angela Don.
Dropping code science at BarCamp.

The format of the course will be pretty much the same as the one I use at Tampa iOS Meetup, where I lead the group through a “code along with me” exercise. I project what’s on my computer on the big screen, and everyone follows along, entering the code as I explain what’s happening.

Since Python has a REPL (Read-Evaluate-Print Loop), I can also have the class go through some exercises and try little coding challenges. It will be a “learn by doing” kind of class.

The main textbooks for the course (which will be provided to students) are Python Crash Course, 2nd Edition…

Book cover: “Python Crash Course, 2nd edition: A Hands-On, Project-Based Introduction to Programming”

…and Automate the Boring Stuff with Python, 2nd edition (which is free to read online):

Book cover: “Automate the Boring Stuff with Python, 2nd edition: Practical Programming for Total Beginners”In order to minimize confusion, we’ll all use the same tools in the course, namely the Anaconda Individual Edition distribution of Python 3.7 and associated tools…Logo: Anaconda…and Visual Studio Code:

Logo: Visual Studio CodeBoth are available free of charge, and run on macOS, Windows, and Linux.

It’ll be fun! Watch this space; I’ll post some snippets from the course as it progresses.

Interested in signing up? Visit Computer Coach’s site and speak to them. Don’t dawdle — it starts tonight!

Current Events Programming What I’m Up To

I’m teaching an online Python programming course!

Photo: Man’s hand on Mac laptop, with Python book on the side. Caption: “Intro to Python course / Starts this Monday!”

Graohic: Computer Coach Training Center logoI’ll be teaching a live online course on Python programming on behalf of Computer Coach Training Center starting Monday. Here are the details:

  • What: Intro to Python Coding course
  • When: Monday and Wednesday evenings, 6:00 – 10:00 p.m., starting Monday, July 13 and ending Wednesday, August 12 (6 weeks, twice a week)
  • Where: Online.
  • How much: $900 — and Computer Coach has grants that can cover the cost if you’re unemployed and based in the Tampa Bay area (contact them to see if you qualify)
  • What you’ll need:
    • A computer that was made sometime in the last ten years. My main computer is a 2014-era MacBook Pro, but I’ll be doing demonstrations on a 2012-era Lenovo ThinkPad running Linux Mint, a 2009-era Compaq laptop running Peppermint Linux, and a $35 Raspberry Pi.
    • An internet connection. This is an online course, after all.

To register for this course, visit this page and tap the Attend Online button. Someone from Computer Coach will contact you.

Screenshot: The Meetup page for the Python course, with the “Attend online” button highlighted.

The course description

Photo: Woman’s hands typing on Mac laptop.

This is an introduction to the Python programming language. Now in the top 10 programming languages according to the TIOBE Programming Language Index, it is versatile enough to have a wide array of uses, from simple scripting to powering Instagram, Spotify, Netflix, Dropbox, and more. Its combination of simplicity and vast scientific and math libraries have made it the preferred programming language for data science and machine learning. If you’re looking for a first programming language, Python is an excellent choice.


This is not a passive course! This isn’t the kind of course where the instructor lectures over slides while you take notes (or pretend to take notes while surfing the web or checking your social media feeds). In this course, you’ll be actively taking part in the learning process, entering code, experimenting, making mistakes, correcting those mistakes, and producing working applications. You will learn by doing. At the end of each session, you’ll have a collection of little Python programs that you wrote, and which you can use as the basis for your own work.

The course will start at the most basic level by walking you through the process of downloading and installing the necessary tools to start Python programming. From there, you’ll learn the building blocks of the Python programming language:

  • Control structures that determine what your programs do,
  • Data structures to store the information that your programs act on,
  • Functions and objects to organize your code, and
  • Using libraries as building blocks for your applications.

You’ll write all sorts of programs…

  • You’ll use Python in “immediate mode” to perform quick calculations (and you’ll sharpen your command-line skills in the process).
  • You’ll write scripts to simplify or automate tedious tasks.
  • You’ll build web applications.
  • And since it’s a networked, data-driven world where no application is an island, you’ll learn how to use Python to interact with web services and databases.

Better still, you’ll learn how to think like a programmer. You’ll learn how to look at a goal and learn how you could write a program to meet it, and how that program could be improved or enhanced. You’ll learn skills that will serve you well as you take up other programming languages, and even learn a little bit about the inner workings of computers, operating systems, and the internet.



Converting a number into words, this time with Python and

Teaching a person how to spell out numbers involves a lot of repetition. Tampa Bay’s own Jack Hartmann, whose children’s educational YouTube channel has over a million subscribers and 300 million views, knows this. He’s got a video that teaches kids the words for the numbers 0 through 10:

Don’t underestimate the power of videos for kids — Jack’s laughing all the way to the bank. This online estimator says that his YouTube channel should be earning about $70,000 every month, and keep in mind that his particular line of work has probably benefited from everyone being stuck at home. I may have to do something similar with the accordion when this software fad passes.

If you just wanted to be able to convert any number from 0 through 10 into word form in Python, you could use a list…

…and if you wanted the number 3 in word form, you’d use this:

You wouldn’t want to take this approach for a larger set of numbers, and you probably wouldn’t want to code it yourself. Luckily, you don’t have to do this in Python, thanks to the module.

Using is a module that does all sorts of processing to make your programs’ text output grammatically correct. If you hate seeing output like this…

You have 1 items in your cart.

…or this…

You have a egg in your inventory.

…you can use to automatically use the correct singular or plural form, use “a” or “an” when appropriate, and so much more.

(I’ll cover in greater detail in a future article.)

In addition to all these grammatical goodies, can also be used to convert numbers to words.

To use, you’ll need to install it first. The simplest way to do so is with pip:

Once installed, you can use it in your Python programs. Here’s an example:

It produces this output:

fifty-four thousand, three hundred and twenty-one

The number_to_words() method has a number of optional parameters that are useful in certain circumstances. For instance, there’s the boolean wantlist parameter, which causes the word output to be broken into “chunks”:

It produces this output:

[‘fifty-four thousand’, ‘three hundred and twenty-one’]

Suppose you want the number to be converted into its individual digits as words. You’d use the group parameter:

What if you’re using the group parameter set to 1, but want to get all UK English and have it use the word “naught” for zero? Or maybe you want your program to sound like a film noir gangster and say “zip” instead? Or you want it recite a phone number and say “oh”? That’s what the zero parameter is for:

The one parameter does the same thing, but for the digit 1:

Want to get all Star Trek? Use the decimal parameter to change the default decimal word to “mark”.

A lot of style guides tell you to spell out the numbers zero through ten, and use the number form for numbers 11 and greater. The threshold parameter makes this easy:

Go ahead — import and play with it. There’s a lot of power in that module, and it goes way beyond just converting words to numbers!

Also worth checking out

If you’re an iOS/macOS programmer, you’ll want to look at the previous article, Converting a number from a numeric form into words in just four lines of Swift.

Career Programming

Programmer interview challenge 2, part 4: Using “watchers” to play FizzBuzz “properly”

The Marvel Comics character known as “The Watcher” - “I am known as The WATCHER. My sworn task is to observe and chronicle great events within this sector of the universe. My curse is to always witness and never participate. I must be true to this duty, even to the brink of Armageddon!”
Marvel Comics’ Watcher. His people are sworn to observe, but not interfere in, everything that happens in the universe.

After reading the previous article on FizzBuzz solutions, Reginald “raganwald” Braithwaite, whom I know from my days as part of the Toronto tech scene, tweeted this:

JenniferPlusPlus agrees:

They both make a good point. If you’re playing the FizzBuzz game as the original children’s game and not as an exercise to prove that you can actually write a program, you’d do it like this:

  • Players sit in a circle, not unlike “the circle” in That ’70s Show.(Players don’t have to be high, unless it helps.)
  • The player designated to go first says the number 1, and each player afterwards counts one number in turn. The next player in the circle says 2, and so on.
  • However, for every third number, instead of calling out the number, the player whose turn it is should say “Fizz”.
  • …and for every fifth number, instead of calling out the number, the player whose turn it is should say “Buzz”.
  • The “Fizz” and “Buzz” rules, as the kids would say, stack. In other words, for every number that is both the third and fifth, the player needs to say “Fizz” followed by “Buzz”, or “FizzBuzz”.

So in the spirit of the original game, I’ve put together a FizzBuzz solution that uses “watchers” to keep track of  “every xth number”, with one watcher to keep track of when it’s time to say “Fizz”, and another for when it’s time to say “Buzz”. When it’s time to say “FizzBuzz”, they’ll work in tandem.

I created a class called WordWatcher, which can be summarized as shown below:

Here’s its code:

Some notes about this code:

    • For those of you who aren’t familiar with Python’s approach to class methods, the first parameter for every method in a class is self. It’s the one parameter you don’t fill when calling a method, because Python calls it implicitly (seemingly in violation of Python’s general guideline that explicit is better than implicit). There’s a reason behind this, and it’s explained in this article: Understanding self in Python.
    • Also note that instance variables are declared and defined in the initializer method, __init__(), and any reference to them is always preceded by self.
    • The observe_next_turn() method is meant to be called as the fizzBuzz method proceeds to each new number. It updates the watcher’s internal counter and sets the time_for_word flag accordingly.
    • The speak() method outputs the watcher’s word if it’s time to say the word, or an empty string otherwise.

For FizzBuzz, we’ll need to create two watchers:

  1. One to keep watch for every third turn, at which point it should say “Fizz”, and
  2. one to keep watch for every third turn, at which point it should say “Buzz”.

With the WordWatcher class defined, we can create these two watchers like so:

It will become handy to have these two watchers in the same place. Since the “ha ha only serious” joke about Python is that everything is a list, let’s put them into a list:

Let’s define a fizzBuzz() function that makes use of this list of word watchers:

If you’ve been following the FizzBuzz series of articles, most of this code will be familiar. The part that might need explaining is the part with the comment “This part might need some explaining”.

Explaining the part that needs explaining

Let’s look at the first of the three lines of code in that part:

  • The _ on the left side of the = sign is a throwaway variable. It says “I don’t care about what you do on the other side of the = sign; only that you do something on the other side of the = sign”.
  • On the right side of the= sign is a list comprehension, which is Python’s “show, don’t tell” way of building lists. This list comprehension simply says “call the observe_next_turn() method of every object in the list”.

Let’s look at the next line:

  • This line creates a map that converts the watchers in the list into the words they should say for this turn. If the current turn means that it’s time for any one of them to speak, the watcher will be mapped to the word it’s supposed to say. Otherwise, it will be mapped to an empty string.

And now, the final line:

  • For some reason, map() comes built into Python, but you have to import the functools library in order to use map()’s partner in crime, reduce(). Remember reduce() is a functional programming thingy that takes a collection of items, performs some kind of calculation on that collection, and returns a single value (which you might call a reduction of the collection).
  • The first argument that I’ve provided to reduce() is a lambda — a small function that isn’t given a name — that simply takes the current item in the list and adds it to the previous collected items. Applied over the entire list, it builds a “total”, which in this case is all the words output by the watchers’ speak() methods concatenated together.
  • The second argument is the words map converted into a list. This is the list that the reduce() method will operate on.

At the end of those three lines, current_result will contain one of the following:

  • The empty string
  • Fizz
  • Buzz
  • FizzBuzz

If current_result is still empty at this point, it means that it’s not time for any of the watchers’ words. If this is the case, the string version of the current number is concatenated to current_result:

Here’s the code in its entirety:

And for completeness’ sake, here’s the test file:

You can download and here (2KB, zipped folder with 2 Python files).

That’s a lot of fuss for Fizzbuzz. Why did you do all that?

  1. Reginald asked me to, and I’ve known and respected him for ages, and JenniferPlusPlus seconded the request.
  2. Wait until you see what customers ask you to do.

Did any of this stuff fly over your head?

  1. Don’t feel bad. I had the same trouble when I first learned functional programming, and that was back in 1991, when the computers that ran functional language interpreters were in labs. I spent a lot of time in Queen’s University’s DEClab, which was full of machines that were cutting edge at the time made by a vendor that no longer exists. Computer time, as well as info on any kind of programming, never mind functional programming, was a lot harder to come by. (In case you were wondering, the language we learned was Miranda.)
  2. If you’ve never worked in Python, some of it can be quite weird. It does eventually make sense.
  3. Let me know, either via email or in the comments, if there’s anything you’d like me to cover in greater detail.

Do you have an alternate solution?

I’ve love to hear about it and present it here! Again, let me know via email or in the comments.

What’s next

An elegant JavaScript implementation.

Previously, in the “Programmer interview challenge” series

Career Programming

Programmer interview challenge 2, part 3: FizzBuzz, minus the modulo operator, plus grit

The modulo operator

The standard FizzBuzz solution relies on the modulo operator, which it uses to determine if a number is a multiple of 3 or 5.

If you have a math, computer science, or engineering background, the odds are good that you encountered the modulo operator in your studies, as your courses tended to take a mathematical approach to  programming. (Remember relational calculus from your intro to databases course?)

If you came into programming from some other field and really got into it because you have a knack for problem-solving, you might not be aware of modulo math. That doesn’t mean that you can’t come up with a FizzBuzz solution.

FizzBuzz minus the modulo operator

When you present the FizzBuzz challenge to a large enough group of programmers — typically a dozen or more — there will be a very determined person who will insist that you provide them with no hints whatsoever. It happens.

When that happens, there’s invariably someone who’s either never heard of the modulo operator (%, which returns the remainder of a division operation) or has forgotten it exists. I’ve also seen a competition comprising quick programming challenges where contestants were told to implement FizzBuzz, but without using modulo.

The more mathematically-inclined will use a method like this:

multiple_of_n() determines if a number n is a multiple of a factor f if the result of n / f is the same as n / f with the fractional part removed.

Occasionally, you’ll run into programmers who are unaware that there are functions to remove the fractional part of a number, either through rounding or truncation. Some of them make up for their lack of math background with a combination of creativity and grit.

I’ve seen one solution that looked something like this:

This function turns the given number into a string, isolates the rightmost character of that string, and then returns True if that character is “0” or “5”, which is true for the string form of numbers that are multiples of 5.

My reaction:

That was nothing compared to one method I saw that someone cobbled together to determine if a number was a multiple of 3. They remembered the old grade-school rule that if you add the digits of a number and the total is a multiple of 3, then the number is a multiple of 3.

Based on that, they wrote something like this:

Again, this function starts by converting the given number into a string. It then iterates through that string character by character, turning each character into a number and adding it to a running total. It then checks to see if that total is in a list of multiples of 3.

Since the standard FizzBuzz challenge is supposed to be performed on the numbers 1 through 100, the largest multiple of 3 will be 99, and the sum of its digits will be 18. Hence the list of multiples of 3 starts with 3 and ending with 18.

My reaction:

But hey, it works!

FizzBuzz plus grit

I’ve seen a handful of people with bachelors’ and even masters’ degrees in computer science face the FizzBuzz test and completely fail to produce working code. I’ve been more impressed by the self-taught coders who, in spite of not knowing about the modulo operator, charge head-first into the problem and solve it. These non-modulo solutions might cause a mathematician to react like this…

…but I think that they’re a sign of grit, which is an important quality for a programmer. These people took what they knew, applied a little creativity, and solved a problem that they shouldn’t have been able to solve. They remind me of a line from aviation pioneer Igor Sikorsky:

According to the laws of aerodynamics, the bumblebee can’t fly, but the bumblebee doesn’t know the laws of aerodynamics, so it goes ahead and flies.

Sooner or later, if you’re working on applications that actually matter, you’re going to run into seemingly insurmountable problems. There will always be the fear that something is just too hard to do. Impostor syndrome may rear its ugly head. Developing grit — and yes, it can be developed — is important, and it’s a quality I look for when forming a team.

What’s next

Using “watchers” to play FizzBuzz “properly”. 

Previously, in the “Programmer interview challenge” series