This post recommends calling pygame.display.flip from a thread, which I tested extensively on mac, windows, and linux before posting, but after some feedback from readers, I realize that this strategy is not in fact cross-platform; specifically, the nvidia drivers on linux appear to either crash or display a black window if you try to do this. The SDL FAQ does say that you can’t call “video functions” from multiple threads, and flip does do that under the hood. I do plan to update this post again, either with a method to make it safe, or a method to use slightly more complex timing heuristics to accomplish the same thing. In the meanwhile, please be aware that this may cause portability problems for your code.

I’ve written about this before, but in that context I was writing mainly about frame-rate independence, and only gave a brief mention of vertical sync; the title also mentioned Twisted, and upon re-reading it I realized that many folks who might get a lot of use out of its technique would not have bothered to read it, just because I made it sound like an aside in the context of an animation technique in a game that already wanted to use Twisted for some reason, rather than a comprehensive best practice. Now that Pygame 2.0 is out, though, and the vsync=1 flag is more reliably available to everyone, I thought it would be worth revisiting.

Per the many tutorials out there, including the official one, most Pygame mainloops look like this:

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pygame.display.set_mode((320, 240))

while 1:
    for event in pygame.event.get():
        handleEvent(event)
    for drawable in myDrawables:
        drawable.draw()
    pygame.display.flip()

Obviously that works okay, or folks wouldn’t do it, but it can give an impression of a certain lack of polish for most beginner Pygame games.

The thing that’s always bothered me personally about this idiom is: where does the networking go? After spending many years trying to popularize event loops in Python, I’m sad to see people implementing loops over and over again that have no way to get networking, or threads, or timers scheduled in a standard way so that libraries could be written without the application needing to manually call them every frame.

But, who cares how I feel about it? Lots of games don’t have networking¹. There are more general problems with it. Specifically, it is likely to:

1. waste power, and
2. look bad.

Wasting Power

Why should anyone care about power when they’re making a video game? Aren’t games supposed to just gobble up CPUs and GPUs for breakfast, burning up as much power as they need for the most gamer experience possible?

Chances are, if you’re making a game that you expect anyone that you don’t personally know to play, they’re going to be playing it on a laptop². Pygame might have a reputation for being “slow”, but for a simple 2D game with only a few sprites, Python can easily render several thousand frames per second. Even the fastest display in the world can only refresh at 360Hz³. That’s less than one thousand frames per second. The average laptop display is going to be more like 60Hz, or — if you’re lucky — maybe 120. By rendering thousands of frames that the user never even sees, you warm up their CPU uncomfortably⁴, and you waste 10x (or more) of their battery doing useless work.

At some point your game might have enough stuff going on that it will run the CPU at full tilt, and if it does, that’s probably fine; at least then you’ll be using up that heat and battery life in order to make their computer do something useful. But even if it is, it’s probably not doing that all of the time, and battery is definitely a use-over-time sort of problem.

Looking Bad

If you’re rendering directly to the screen without regard for vsync, your players are going to experience Screen Tearing, where the screen is in the middle of updating while you’re in the middle of drawing to it. This looks especially bad if your game is panning over a background, which is a very likely scenario for the usual genre of 2D Pygame game.

How to fix it?

Pygame lets you turn on VSync, and in Pygame 2, you can do this simply by passing the pygame.SCALED flag and the vsync=1 argument to set_mode().

Now your game will have silky smooth animations and scrolling⁵! Solved!

But... if the fix is so simple, why doesn’t everybody — including, notably, the official documentation — recommend doing this?

The solution creates another problem: pygame.display.flip may now block until the next display refresh, which may be many milliseconds.

Even worse: note the word “may”. Unfortunately, behavior of vsync is quite inconsistent between platforms and drivers, so for a properly cross-platform game it may be necessary to allow the user to select a frame rate and wait on an asyncio.sleep than running flip in a thread. Using the techniques from the answers to this stack overflow answer you can establish a reasonable heuristic for the refresh rate of the relevant display, but if adding those libraries and writing that code is too complex, “60” is probably a good enough value to start with, even if the user’s monitor can go a little faster. This might save a little power even in the case where you can rely on flip to tell you when the monitor is actually ready again; if your game can only reliably render 60FPS anyway because there’s too much Python game logic going on to consistently go faster, it’s better to achieve a consistent but lower framerate than to be faster but inconsistent.

The potential for blocking needs to be dealt with though, and it has several knock-on effects.

For one thing, it makes my “where do you put the networking” problem even worse: most networking frameworks expect to be able to send more than one packet every 16 milliseconds.

More pressingly for most Pygame users, however, it creates a minor performance headache. You now spend a bunch of time blocked in the now-blocking flip call, wasting precious milliseconds that you could be using to do stuff unrelated to drawing, like handling user input, updating animations, running AI, and so on.

The problem is that your Pygame mainloop has 3 jobs:

1. drawing
2. game logic (AI and so on)
3. input handling

What you want to do to ensure the smoothest possible frame rate is to draw everything as fast as you possibly can at the beginning of the frame and then call flip immediately to be sure that the graphics have been delivered to the screen and they don’t have to wait until the next screen-refresh. However, this is at odds with the need to get as much done as possible before you call flip and possibly block for 1/60th of a second.

So either you put off calling flip, potentially risking a dropped frame if your AI is a little slow, or you call flip too eagerly and waste a bunch of time waiting around for the display to refresh. This is especially true of things like animations, which you can’t update before drawing, because you have to draw this frame before you worry about the next one, but waiting until after flip wastes valuable time; by the time you are starting your next frame draw, you possibly have other code which now needs to run, and you’re racing to get it done before that next flip call.

Now, if your Python game logic is actually saturating your CPU — which is not hard to do — you’ll drop frames no matter what. But there are a lot of marginal cases where you’ve mostly got enough CPU to do what you need to without dropping frames, and it can be a lot of overhead to constantly check the clock to see if you have enough frame budget left to do one more work item before the frame deadline - or, for that matter, to maintain a workable heuristic for exactly when that frame deadline will be.

The technique to avoid these problems is deceptively simple, and in fact it was covered with the deferToThread trick presented in my earlier post. But again, we’re not here to talk about Twisted. So let’s do this the no-additional-dependencies, stdlib-only way, with asyncio:

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import asyncio
import time
from math import inf

from pygame.display import set_mode, flip
from pygame.constants import SCALED
from pygame.event import get

event_handler = ...
drawables = [...]

async def pygame_loop(framerate_limit=inf):
    loop = asyncio.get_event_loop()
    screen_surface = set_mode(size=(480, 255), flags=SCALED, vsync=1)
    next_frame_target = 0.0
    limit_frame_duration = (1.0 / framerate_limit)

    while True:

        if limit_frame_duration:
            # framerate limiter
            this_frame = time.time()
            delay = next_frame_target - this_frame
            if delay > 0:
                await asyncio.sleep(delay)
            next_frame_target = this_frame + limit_frame_duration

        for drawable in drawables:
            drawable.draw(screen_surface)
        events_to_handle = list(get())
        events_handled = loop.create_task(handle_events(events_to_handle))
        await loop.run_in_executor(None, flip)
        # don’t want to accidentally start drawing again until events are done
        await events_handled

async def handle_events(events_to_handle):
    # note that this must be an async def even if it doesn’t await
    for event in events_to_handle:
        event_handler.handle_event(event)

asyncio.run(pygame_loop(120))

Go Forth and Loop Better

At some point I will probably release my own wrapper library⁶ which does something similar to this, but I really wanted to present this as a technique rather than as some packaged-up code to use, since do-it-yourself mainloops, and keeping dependencies to a minimum, are such staples of Pygame community culture.

As you can see, this technique is only a few lines longer than the standard recipe for a Pygame main loop, but you now have access to a ton of additional functionality:

• You can manage your framerate independence in both animations and game logic by just setting some timers and letting the frames update at the appropriate times; stop worrying about doing math on the clock by yourself!
• Do you want to add networked multiplayer? No problem! Networking all happens inside the event loop, make whatever network requests you want, and never worry about blocking the game’s drawing on a network request!
• Now your players’ laptops run cool while playing, and the graphics don’t have ugly tearing artifacts any more!

I really hope that this sees broader adoption so that the description “indie game made in Python” will no longer imply “runs hot and tears a lot when the screen is panning”. I’m also definitely curious to hear from readers, so please let me know if you end up using this technique to good effect!⁷

One of my favorite features within Twisted — but also one of the least known — is LoopingCall.withCount, which can be used in applications where you have some real-time thing happening, which needs to keep happening at a smooth rate regardless of any concurrent activity or pauses in the main loop. Originally designed for playing audio samples from a softphone without introducing a desync delay over time, it can also be used to play animations while keeping track of their appropriate frame.

LoopingCall is all around a fun tool to build little game features with. I’ve built a quick little demo to showcase some discoveries I’ve made over a few years of small hobby projects (none of which are ready for an open-source release) over here: DrawSnek.

This little demo responds to 3 key-presses:

1. q quits. Always a useful thing for full-screen apps which don’t always play nice with C-c :).
2. s spawns an additional snek. Have fun, make many sneks.
3. h introduces a random “hiccup” of up to 1 full second so you can see what happens visually when the loop is overburdened or stuck.

Unfortunately a fully-functioning demo is a bit lengthy to go over line by line in a blog post, so I’ll just focus on a couple of important features for stutter- and tearing-resistant animation & drawing with PyGame & Twisted.

For starters, you’ll want to use a very recent prerelease of PyGame 2, which recently added support for vertical sync even without OpenGL mode; then, pass the vsync=1 argument to set_mode:

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screen = pygame.display.set_mode(
    (640 * 2, 480 * 2),
    pygame.locals.SCALED | pygame.locals.FULLSCREEN,
    vsync=1
)

To allow for as much wall-clock time as possible to handle non-drawing work, such as AI and input handling, I also use this trick:

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def drawScene():
    screen.fill((0, 0, 0))
    for drawable in self.drawables:
        drawable.draw(screen)
    return deferToThread(pygame.display.flip)

LoopingCall(drawScene).start(1 / 62.0)

By deferring pygame.display.flip to a thread¹, the main loop can continue processing AI timers, animation, network input, and user input while blocking and waiting for the vertical blank. Since the time-to-vblank can easily be up to 1/120th of a second, this is a significant amount of time! We know that the draw won’t overlap with flip, because LoopingCall respects Deferreds returned from its callable and won’t re-invoke you until the Deferred fires.

Drawing doesn’t use withCount, because it just needs to repeat about once every refresh interval (on most displays, about 1/60th of a second); the vblank timing is what makes sure it lines up.

However, animation looks like this:

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def animate(self, frameCount):
    self.index += frameCount
    self.index %= len(self.images)

We move the index forward by however many frames it’s been, then be sure it wraps around by modding it by the number of frames.

Similarly, the core² of movement looks like this:

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def move(self, frameCount):
    self.sprite.x += frameCount * self.dx
    self.sprite.y += frameCount * self.dy

Rather than moving based on the number of times we’ve been called, which can result in slowed-down movement when the framerate isn’t keeping up, we jump forward by however many frames we should have been called at this point in time.

One of these days, maybe I’ll make an actual game, but in the meanwhile I hope you all enjoy playing with these fun little basic techniques for using Twisted in your game engine.

I have written a tool you can actually use rather than copying and pasting shell-script snippets, which you can read about in a new post here. I've done my best to update the accuracy of the information below as well, particularly with respect to which Python you want and why, but it is a much older post and I could easily have missed something.

I’ve written and spoken at some length about shipping software in the abstract. Sometimes I’ve even had the occasional concrete tidbit, but that advice wasn’t really complete.

In honor of Eevee’s delightful Games Made Quick???, I’d like to help you package your games even quicker than you made them.

Who is this for?

About ten years ago I made a prototype of a little PyGame thing which I wanted to share with a few friends. Building said prototype was quick and fun, and very different from the usual sort of work I do. But then, the project got just big enough that I started to wonder if it would be possible to share the result, and thus began the long winter of my discontent with packaging tools.

I might be the only one, but... I don’t think so. The history of PyWeek, for example, looks to be a history of games distributed as Github repositories, or, at best, apps which don’t launch. It seems like people who participate in game jams with Unity push a button and publish their games to Steam; people who participate in game jams with Python wander away once the build toolchain defeats them.

So: perhaps you’re also a Python programmer, and you’ve built something with PyGame, and you want to put it on your website so your friends can download it. Perhaps many or most of your friends and family are Mac users. Perhaps you tried to make a thing with py2app once, and got nothing but inscrutable tracebacks or corrupt app bundles for your trouble.

If so, read on and enjoy.

What changed?

If things didn’t work for me when I first tried to do this, what’s different now?

• the packaging ecosystem in general is far less buggy, and py2app’s dependencies, like setuptools, have become far more reliable as well. Many thanks to Donald Stufft and the whole PyPA for that.
• Binary wheels exist, and the community has been getting better and better at building self-contained wheels which include any necessary C libraries, relieving the burden on application authors to figure out gnarly C toolchain issues.
• The PyGame project now ships just such wheels for a variety of Python versions on Mac, Windows, and Linux, which removes a whole huge pile of complexity both in generally understanding the C toolchain and specifically understanding the SDL build process.
• py2app has been actively maintained and many bugs have been fixed - many thanks to Ronald Oussoren et. al. for that.
• I finally broke down and gave Apple a hundred dollars so I can produce an app that normal humans might actually be able to run.

There are still weird little corner cases you have to work around — hence this post – but mostly this is the story of how years of effort by the Python packaging community have resulted in tools that are pretty close to working out of the box now.

Step 0: Development Setup

You will also want to use a virtual environment for development.

Finally: pip install all your requirements into your virtualenv, including PyGame itself.

Step 1: Make an icon

All good apps need an icon, right?

When I was young, one would open up ResEdit Resorcerer MPW CodeWarrior Project Builder Icon Composer Xcode and create a new ICON resource cicn resource .tiff file .icns file. Nowadays there’s some weird opaque stuff with xcassets files and Contents.json and “Copy Bundle Resources” in the default Swift and Objective C project templates and honestly I can’t be bothered to keep track of what’s going on with this nonsense any more.

Luckily the OS ships with the macOS-specific “scriptable image processing system”, which can helpfully convert an icon for you. Make yourself a 512x512 PNG file in your favorite image editor (with an alpha channel!) that you want to use as your icon, then run it something like this:

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$ sips -s format icns Icon.png --out Icon.icns

somewhere in your build process, to produce an icon in the appropriate format.

There’s also one additional wrinkle with PyGame: once you’ve launched the game, PyGame helpfully assigns the cute, but ugly, default PyGame icon to your running process. To avoid this, you’ll need these two lines somewhere in your initialization code, somewhere before pygame.display.init (or, for that matter, pygame.display.<anything>):

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from pygame.sdlmain_osx import InstallNSApplication
InstallNSApplication()

Obviously this is pretty Mac-specific so you probably want this under some kind of platform-detection conditional, perhaps this one.

Step 2: Include All The Dang Files, I Don’t Care About Performance

Unfortunately py2app still tries really hard to jam all your code into a .zip file, which breaks the world in various hilarious ways. Your app will probably have some resources you want to load, as will PyGame itself.

Supposedly, packages=["your_package"] in your setup.py should address this, and it comes with a “pygame” recipe, but neither of these things worked for me. Instead, I convinced py2app to splat out all the files by using the not-quite-public “recipe” plugin API:

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import py2app.recipes
import py2app.build_app

from setuptools import find_packages, setup

pkgs = find_packages(".")

class recipe_plugin(object):
    @staticmethod
    def check(py2app_cmd, modulegraph):
        local_packages = pkgs[:]
        local_packages += ['pygame']
        return {
            "packages": local_packages,
        }

py2app.recipes.my_recipe = recipe_plugin

APP = ['my_main_file.py']
DATA_FILES = []
OPTIONS = {}
OPTIONS.update(
    iconfile="Icon.icns",
    plist=dict(CFBundleIdentifier='com.example.yourdomain.notmine')
)

setup(
    name="Your Game",
    app=APP,
    data_files=DATA_FILES,
    include_package_data=True,
    options={'py2app': OPTIONS},
    setup_requires=['py2app'],
    packages=pkgs,
    package_data={
        "": ["*.gal" , "*.gif" , "*.html" , "*.jar" , "*.js" , "*.mid" ,
             "*.png" , "*.py" , "*.pyc" , "*.sh" , "*.tmx" , "*.ttf" ,
             # "*.xcf"
        ]
    },
)

This is definitely somewhat less efficient than py2app’s default of stuffing the code into a single zip file, but, as a counterpoint to that: it actually works.

Step 3: Build it

Hopefully, at this point you can do python setup.py py2app and get a shiny new app bundle in dist/$NAME.app. We haven’t had to go through the hell of quarantine yet, so it should launch at this point. If it doesn’t, sorry :-(.

You can often debug more obvious fail-to-launch issues by running the executable in the command line, by running ./dist/$NAME.app/Contents/MacOS/$NAME. Although this will run in a slightly different environment than double clicking (it will have all your shell’s env vars, for example, so if your app needs an env var to work it might mysteriously work there) it will also print out any tracebacks to your terminal, where they’ll be slightly easier to find than in Console.app.

Once your app at least runs locally, it’s time to...

Step 4: Code sign it

All the tutorials that I’ve found on how to do this involve doing Xcode project goop where it’s not clear what’s happening underneath. But despite the fact that the introductory docs aren’t quite there, the underlying model for codesigning stuff is totally common across GUI and command-line cases. However, actually getting your cert requires Xcode, an apple ID, and a credit card.

After paying your hundred dollars, go into Xcode, go to Accounts, hit “+”, “Apple ID”, then log in. Then, in your shiny new account, go to “Manage Certificates”, hit the little “+”, and (assuming, like me, you want to put something up on your own website, and not submit to the Mac App Store), and choose Developer ID Application. You probably think you want “mac app distribution” because you are wanting to distribute a mac app! But you don’t.

Next, before you do anything else, make sure you have backups of your certificate and private key. You really don’t want to lose the private key associated with that cert.

Now quit Xcode; you’re done with the GUI.

You will need to know the identifier of your signing key though, which should be output from the command:

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$ security find-identity -v -p codesigning | grep 'Developer ID' | sed -e 's/.*"\(.*\)"/\1/'

You probably want to put that in your build script, since you want to sign with the same identity every time. Further commands here will assume you’ve copied one of the lines of results from that command and done export IDENTITY="..." with it.

Step 4a: Become Aware Of New Annoying Requirements

Update for macOS Catalina: In Catalina, Apple has added a new code-signing requirement; even for apps distributed outside of the app store, they still have to be submitted to and approved by Apple.

In order to be notarized, you will need to codesign not only your app itself, but to also:

1. add the hardened-runtime exception entitlements that allow Python to work, and
2. directly sign every shared library that is part of your app bundle.

So the actual code-signing step is now a little more complicated.

Step 4b: Write An Entitlements Plist That Allows Python To Work

One of the features that notarization is intended to strongly encourage¹ is the “hardened runtime”, a feature of macOS which opts in to stricter run-time behavior designed to stop malware. One thing that the hardened runtime does is to disable writable, executable memory, which is used by JITs, FFIs ... and malware.

Unfortunately, both Python’s built-in ctypes module and various popular bits of 3rd-party stuff that uses cffi, including pyOpenSSL, require writable, executable memory to work. Furthermore, py2app actually imports ctypes during its bootstrapping phase, so you can’t even get your own code to start running to perform any workarounds unless this is enabled. So this is just if you want to use Python, not if your project requires ctypes directly.

To make this long, sad story significantly shorter and happier, you can create an entitlements property list that enables the magical property which allows this to work. It looks like this:

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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>com.apple.security.cs.allow-unsigned-executable-memory</key>
    <true/>
</dict>
</plist>

Subsequent steps assume that you’ve put this into a file called entitleme.plist in your project root.

Step 4c: SIGN ALL THE THINGS

Notarization also requires that all the executable files in your bundle, not just the main executable, are properly code-signed before submitting. So you’ll need to first run the codesign command across all your shared libraries, something like this:

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$ cd dist
$ find "${NAME}.app" -iname '*.so' -or -iname '*.dylib' |
    while read libfile; do
        codesign --sign "${IDENTITY}" \
                 --entitlements ../entitleme.plist \
                 --deep "${libfile}" \
                 --force \
                 --options runtime;
    done;

Then finally, sign the bundle itself.

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$ codesign --sign "${IDENTITY}" \
         --entitlements ../entitleme.plist \
         --deep "${NAME}.app" \
         --force \
         --options runtime;

Now, your app is code-signed.

Step 5: Archive it

The right way to do this is probably to use dmgbuild or something like it, but what I promised here was quick and dirty, not beautiful and best practices.

You have to make a Zip archive that preserves symbolic links. There are a couple of options for this:

• open dist/, then in the Finder window that comes up, right click on the app and “compress” it
• cd dist; zip -yr $NAME.app.zip $NAME.app

Most importantly, if you use the zip command line tool, you must use the -y option. Without it, your downloadable app bundle will be somewhat mysteriously broken even though the one before you zipped it will be fine.

Step 6: Actually The Rest Of Step 4: Request Notarization

Notarization is a 2-step process, which is somewhat resistant to fully automating. You submit to Apple, then they email you the results of doing the notarization, then if that email indicates that your notarization succeded, you can “staple” the successful result to your bundle.

The thing you notarize is an archive, which is why you need to do step 5 first. Then, you need to do this:

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$ xcrun altool --notarize-app \
      --file "${NAME}.app.zip" \
      --type osx \
      --username "${YOUR_DEVELOPER_ID_EMAIL}" \
      --primary-bundle-id="${YOUR_BUNDLE_ID}";

Be sure that YOUR_BUNDLE_ID matches the CFBundleIdentifier you told py2app about before, so that the tool can find your app bundle inside the archive.

You’ll also need to type in the iCloud password for your Developer ID account here.²

Step 6a: Wait A Minute

Anxiously check your email for an hour or so. Hope you don’t get any errors.

Step 6b: Finish Notarizing It, Finally!

Once Apple has a record of the app’s notarization, their tooling will recognize it, so you don’t need any information from the confirmation email or the previous command; just make sure that you are running this on the exact same .app directory you just built and archived and not a version that differs in any way.

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$ xcrun stapler staple "./${NAME}.app";

Finally, you will want to archive it again:

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$ zip -qyr "${NAME}.notarized.app.zip" "${NAME}.app";

Step 7: Download it

Ideally, at this point, everything should be working. But to make sure that code-signing and archiving and notarizing and re-archiving went correctly, you should have either a pristine virtual machine with no dev tools and no Python installed, or a non-programmer friend’s machine that can serve the same purpose. They probably need a relatively recent macOS - my own experience has shown that apps made using the above technique will definitely work on High Sierra (and later) and will definitely break on Yosemite (and earlier); they probably start working at some OS version between those.

There’s no tooling that I know of that can clearly tell you whether your mac app depends on some detail of your local machine. Even for your dependencies, there’s no auditwheel for macOS.

Updated 2019-06-27: It turns out there is an auditwheel like thing for macOS: delocate! In fact, it predated and inspired auditwheel!

Thanks to Nathaniel Smith for the update (which he provided in, uh, January of 2018 and I’ve only just now gotten around to updating...).

Nevertheless, it’s always a good idea to check your final app build on a fresh computer before you announce it.

Coda

If you were expecting to get to the end and download my cool game, sorry to disappoint! It really is a half-broken prototype that is in no way ready for public consumption, and given my current load of personal and professional responsibilities, you definitely shouldn’t expect anything from me in this area any time soon, or, you know, ever.

But, from years of experience, I know that it’s nearly impossible to summon any motivation to work on small projects like this without the knowledge that the end result will be usable in some way, so I hope that this helps someone else set up their Python game-dev pipeline.

I’d really like to turn this into a 3-part series, with a part for Linux (perhaps using flatpak? is that a good thing?) and a part for Windows. However, given my aforementioned time constraints, I don’t think I’m going to have the time or energy to do that research, so if you’ve got the appropriate knowledge, I’d love to host a guest post on this blog, or even just a link to yours.

If this post helped you, if you have questions or corrections, or if you’d like to write the Linux or Windows version of this post, let me know.