Tips And Tricks for Shipping a PyGame App on the Mac

A quick and dirty guide to getting that little PyGame hack you did up and running on someone else’s Mac.

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

Get a good Python. Use Homebrew, and brew install python3. If you need python 2, brew install python2. Don’t use the System python. Probably nothing will work.

You probably also want to use a virtualenv for development. This post is about how to build a for-real thing that other people can download, but part of the magic of Python is the interactive, real-time dynamic nature of everything. Running the full build pipeline every time you change a file or an asset is slow and annoying. However, there’s a weird thing where certain parts of the macOS GUI won’t work right (in PyGame’s case, mostly keyboard focus) unless your code appears to be in an application bundle.

I made this dumb little thing which lets you fake out enough of this that the OS won’t hassle you: you just need to pip install venvdotapp; venvdotapp inside the virtualenv where you’re making your pygame app.

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, you just popped over into ResEdit Resorcerer MPW CodeWarrior Project Builder Icon Composer Xcode and created 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:

1
$ 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>):

1
2
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: Just 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 just splat out all the files by using the not-quite-public “recipe” plugin API:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
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 just 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 just 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:

1
$ 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. The command to do the signing is:

1
$ codesign -fs "${YOUR_DEVELOPER_ID_IDENTITY}" --deep "dist/${NAME}.app"

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 an archive that preserves symbolic links. There are a few options for this:

  • open dist/, then in the Finder window that comes up, right click on the app and “compress” it
  • cd dist; tar czf $NAME.app.tgz $NAME.app
  • 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: Download it

Ideally, at this point, everything should be working. But to make sure that code-signing and 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 - I know that apps made using the above technique will definitely work on High Sierra and will definitely break on Yosemite; 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. So 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.

Careful With That PyPI

PyPI credentials are important. Here are some tips for securing them a little better.

Too Many Secrets

A wise man once said, “you shouldn’t use ENV variables for secret data”. In large part, he was right, for all the reasons he gives (and you should read them). Filesystem locations are usually a better operating system interface to communicate secrets than environment variables; fewer things can intercept an open() than can read your process’s command-line or calling environment.

One might say that files are “more secure” than environment variables. To his credit, Diogo doesn’t, for good reason: one shouldn’t refer to the superiority of such a mechanism as being “more secure” in general, but rather, as better for a specific reason in some specific circumstance.

Supplying your PyPI password to tools you run on your personal machine is a very different case than providing a cryptographic key to a containerized application in a remote datacenter. In this case, based on the constraints of the software presently available, I believe an environment variable provides better security, if you use it correctly.

Popping A Shell By Any Other Name

If you upload packages to the python package index, and people use those packages, your PyPI password is an extremely high-privilege credential: effectively, it grants a time-delayed arbitrary code execution privilege on all of the systems where anyone might pip install your packages.

Unfortunately, the suggested mechanism to manage this crucial, potentially world-destroying credential is to just stick it in an unencrypted file.

The authors of this documentation know this is a problem; the authors of the tooling know too (and, given that these tools are all open source and we all could have fixed them to be better about this, we should all feel bad).

Leaving the secret lying around on the filesystem is a form of ambient authority; a permission you always have, but only sometimes want. One of the worst things about this is that you can easily forget it’s there if you don’t use these credentials very often.

The keyring is a much better place, but even it can be a slightly scary place to put such a thing, because it’s still easy to put it into a state where some random command could upload a PyPI release without prompting you. PyPI is forever, so we want to measure twice and cut once.

Luckily, even more secure places exist: password managers. If you use https://1password.com or https://www.lastpass.com, both offer command-line interfaces that integrate nicely with PyPI. If you use 1password, you’ll really want https://stedolan.github.io/jq/ (apt-get install jq, brew install jq) to slice & dice its command-line.

The way that I manage my PyPI credentials is that I never put them on my filesystem, or even into my keyring; instead, I leave them in my password manager, and very briefly toss them into the tools that need them via an environment variable.

First, I have the following shell function, to prevent any mistakes:

1
2
3
4
function twine () {
    echo "Use dev.twine or prod.twine depending on where you want to upload.";
    return 1;
}

For dev.twine, I configure twine to always only talk to my local DevPI instance:

1
2
3
4
5
6
function dev.twine () {
    env TWINE_USERNAME=root \
        TWINE_PASSWORD= \
        TWINE_REPOSITORY_URL=http://127.0.0.1:3141/root/plus/ \
        twine "$@";
}

This way I can debug Twine, my setup.py, and various test-upload things without ever needing real credentials at all.

But, OK. Eventually, I need to actually get the credentials and do the thing. How does that work?

1Password

1password’s command line is a little tricky to log in to (you have to eval its output, it’s not just a command), so here’s a handy shell function that will do it.

1
2
3
4
5
6
function opme () {
    # Log this shell in to 1password.
    if ! env | grep -q OP_SESSION; then
        eval "$(op signin "$(jq -r '.latest_signin' ~/.op/config)")";
    fi;
}

Then, I have this little helper for slicing out a particular field from the OP JSON structure:

1
2
3
function _op_field () {
    jq -r '.details.fields[] | select(.name == "'"${1}"'") | .value';
}

And finally, I use this to grab the item I want (named, memorably enough, “PyPI”) and invoke Twine:

1
2
3
4
5
6
7
function prod.twine () {
    opme;
    local pypi_item="$(op get item PyPI)";
    env TWINE_USERNAME="$(echo ${pypi_item} | _op_field username)" \
        TWINE_PASSWORD="$(echo "${pypi_item}" | _op_field password)" \
        twine "$@";
}

LastPass

For lastpass, you can just log in (for all shells; it’s a little less secure) via lpass login; if you’ve logged in before you often don’t even have to do that, and it will just prompt you when running command that require you to be logged in; so we don’t need the preamble that 1password’s command line did.

Its version of prod.twine looks quite similar, but its plaintext output obviates the need for jq:

1
2
3
4
5
function prod.twine () {
    env TWINE_USERNAME="$(lpass show PyPI --username)" \
        TWINE_PASSWORD="$(lpass show PyPI --password)" \
        twine "$@";
}

In Conclusion

“Keep secrets out of your environment” is generally a good idea, and you should always do it when you can. But, better a moment in your process environment than an eternity on your filesystem. Environment-based configuration can be a very useful stopgap for limiting the lifetimes of credentials when your tools don’t support more sophisticated approaches to secret storage.1

Post Script

If you are interested in secure secret storage, my micro-project secretly might be of interest. Right now it doesn’t do a whole lot; it’s just a small wrapper around the excellent keyring module and the pinentry / pinentry-mac password prompt tools. secretly presents an interface both for prompting users for their credentials without requiring the command-line or env vars, and for saving them away in keychain, for tools that need to pull in an API key and don’t want to make the user manually edit a config file first.


  1. Really, PyPI should have API keys that last for some short amount of time, that automatically expire so you don’t have to freak out if you gave somebody a 5-year-old laptop and forgot to wipe it first. But again, if I wanted that so bad, I should have implemented it myself... 

The Sororicide Antipattern

Don’t murder your parents or your siblings to get their attributes.

Composition is better than inheritance.”. This is a true statement. “Inheritance is bad.” Also true. I’m a well-known compositional extremist. There’s a great talk you can watch if I haven’t talked your ear off about it already.

Which is why I was extremely surprised in a recent conversation when my interlocutor said that while inheritance might be bad, composition is worse. Once I understood what they meant by “composition”, I was even more surprised to find that I agreed with this assertion.

Although inheritance is bad, it’s very important to understand why. In a high-level language like Python, with first-class runtime datatypes (i.e.: user defined classes that are objects), the computational difference between what we call “composition” and what we call “inheritance” is a matter of where we put a pointer: is it on a type or on an instance? The important distinction has to do with human factors.

First, a brief parable about real-life inheritance.


You find yourself in conversation with an indolent heiress-in-waiting. She complains of her boredom whiling away the time until the dowager countess finally leaves her her fortune.

“Inheritance is bad”, you opine. “It’s better to make your own way in life”.

“By George, you’re right!” she exclaims. You weren’t expecting such an enthusiastic reversal.

“Well,”, you sputter, “glad to see you are turning over a new leaf”.

She crosses the room to open a sturdy mahogany armoire, and draws forth a belt holstering a pistol and a menacing-looking sabre.

“Auntie has only the dwindling remnants of a legacy fortune. The real money has always been with my sister’s manufacturing concern. Why passively wait for Auntie to die, when I can murder my dear sister now, and take what is rightfully mine!”

Cinching the belt around her waist, she strides from the room animated and full of purpose, no longer indolent or in-waiting, but you feel less than satisfied with your advice.

It is, after all, important to understand what the problem with inheritance is.


The primary reason inheritance is bad is confusion between namespaces.

The most important role of code organization (division of code into files, modules, packages, subroutines, data structures, etc) is division of responsibility. In other words, Conway’s Law isn’t just an unfortunate accident of budgeting, but a fundamental property of software design.

For example, if we have a function called multiply(a, b) - its presence in our codebase suggests that if someone were to want to multiply two numbers together, it is multiply’s responsibility to know how to do so. If there’s a problem with multiplication, it’s the maintainers of multiply who need to go fix it.

And, with this responsibility comes authority over a specific scope within the code. So if we were to look at an implementation of multiply:

1
2
3
def multiply(a, b):
    product = a * b
    return product

The maintainers of multiply get to decide what product means in the context of their function. It’s possible, in Python, for some other funciton to reach into multiply with frame objects and mangle the meaning of product between its assignment and return, but it’s generally understood that it’s none of your business what product is, and if you touch it, all bets are off about the correctness of multiply. More importantly, if the maintainers of multiply wanted to bind other names, or change around existing names, like so, in a subsequent version:

1
2
3
4
5
def multiply(a, b):
    factor1 = a
    factor2 = b
    result = a * b
    return result

It is the maintainer of multiply’s job, not the caller of multiply, to make those decisions.

The same programmer may, at different times, be both a caller and a maintainer of multiply. However, they have to know which hat they’re wearing at any given time, so that they can know which stuff they’re still repsonsible for when they hand over multiply to be maintained by a different team.

It’s important to be able to forget about the internals of the local variables in the functions you call. Otherwise, abstractions give us no power: if you have to know the internals of everything you’re using, you can never build much beyond what’s already there, because you’ll be spending all your time trying to understand all the layers below it.

Classes complicate this process of forgetting somewhat. Properties of class instances “stick out”, and are visible to the callers. This can be powerful — and can be a great way to represent shared data structures — but this is exactly why we have the ._ convention in Python: if something starts with an underscore, and it’s not in a namespace you own, you shouldn’t mess with it. So: other._foo is not for you to touch, unless you’re maintaining type(other). self._foo is where you should put your own private state.

So if we have a class like this:

1
2
3
class A(object):
    def __init__(self):
        self._note = "a note"

we all know that A()._note is off-limits.

But then what happens here?

1
2
3
4
class B(A):
    def __init__(self):
        super().__init__()
        self._note = "private state for B()"

B()._note is also off limits for everyone but B, except... as it turns out, B doesn’t really own the namespace of self here, so it’s clashing with what A wants _note to mean. Even if, right now, we were to change it to _note2, the maintainer of A could, in any future release of A, add a new _note2 variable which conflicts with something B is using. A’s maintainers (rightfully) think they own self, B’s maintainers (reasonably) think that they do. This could continue all the way until we get to _note7, at which point it would explode violently.


So that’s why Inheritance is bad. It’s a bad way for two layers of a system to communicate because it leaves each layer nowhere to put its internal state that the other doesn’t need to know about. So what could be worse?

Let’s say we’ve convinced our junior programmer who wrote A that inheritance is a bad interface, and they should instead use the panacea that cures all inherited ills, composition. Great! Let’s just write a B that composes in an A in a nice clean way, instead of doing any gross inheritance:

1
2
3
4
class Bprime(object):
    def __init__(self, a):
        for var in dir(a):
            setattr(self, var, getattr(a, var))

Uh oh. Looks like composition is worse than inheritance.


Let’s enumerate some of the issues with this “solution” to the problem of inheritance:

  • How do we know what attributes Bprime has?
  • How do we even know what type a is?
  • How is anyone ever going to grep for relevant methods in this code and have them come up in the right place?

We briefly reclaimed self for Bprime by removing the inheritance from A, but what Bprime does in __init__ to replace it is much worse. At least with normal, “vertical” inheritance, IDEs and code inspection tools can have some idea where your parents are and what methods they declare. We have to look aside to know what’s there, but at least it’s clear from the code’s structure where exactly we have to look aside to.

When faced with a class like Bprime though, what does one do? It’s just shredding apart some apparently totally unrelated object, there’s nearly no way for tooling to inspect this code to the point that they know where self.<something> comes from in a method defined on Bprime.

The goal of replacing inheritance with composition is to make it clear and easy to understand what code owns each attribute on self. Sometimes that clarity comes at the expense of a few extra keystrokes; an __init__ that copies over a few specific attributes, or a method that does nothing but forward a message, like def something(self): return self.other.something().

Automatic composition is just lateral inheritance. Magically auto-proxying all methods1, or auto-copying all attributes, saves a few keystrokes at the time some new code is created at the expense of hours of debugging when it is being maintained. If readability counts, we should never privilege the writer over the reader.


  1. It is left as an exercise for the reader why proxyForInterface is still a reasonably okay idea even in the face of this criticism.2 

  2. Although ironically it probably shouldn’t use inheritance as its interface. 

Python Packaging Is Good Now

setup.py is your friend. It’s real sorry about what happened last time.

Okay folks. Time’s up. It’s too late to say that Python’s packaging ecosystem terrible any more. I’m calling it.

Python packaging is not bad any more. If you’re a developer, and you’re trying to create or consume Python libraries, it can be a tractable, even pleasant experience.

I need to say this, because for a long time, Python’s packaging toolchain was … problematic. It isn’t any more, but a lot of people still seem to think that it is, so it’s time to set the record straight.

If you’re not familiar with the history it went something like this:

The Dawn

Python first shipped in an era when adding a dependency meant a veritable Odyssey into cyberspace. First, you’d wait until nobody in your whole family was using the phone line. Then you’d dial your ISP. Once you’d finished fighting your SLIP or PPP client, you’d ask a netnews group if anyone knew of a good gopher site to find a library that could solve your problem. Once you were done with that task, you’d sign off the Internet for the night, and wait about 48 hours too see if anyone responded. If you were lucky enough to get a reply, you’d set up a download at the end of your night’s web-surfing.

pip search it wasn’t.

For the time, Python’s approach to dependency-handling was incredibly forward-looking. The import statement, and the pluggable module import system, made it easy to get dependencies from wherever made sense.

In Python 2.01, Distutils was introduced. This let Python developers describe their collections of modules abstractly, and added tool support to producing redistributable collections of modules and packages. Again, this was tremendously forward-looking, if somewhat primitive; there was very little to compare it to at the time.

Fast forwarding to 2004; setuptools was created to address some of the increasingly-common tasks that open source software maintainers were facing with distributing their modules over the internet. In 2005, it added easy_install, in order to provide a tool to automate resolving dependencies and downloading them into the right locations.

The Dark Age

Unfortunately, in addition to providing basic utilities for expressing dependencies, setuptools also dragged in a tremendous amount of complexity. Its author felt that import should do something slightly different than what it does, so installing setuptools changed it. The main difference between normal import and setuptools import was that it facilitated having multiple different versions of the same library in the same program at the same time. It turns out that that’s a dumb idea, but in fairness, it wasn’t entirely clear at the time, and it is certainly useful (and necessary!) to be able to have multiple versions of a library installed onto a computer at the same time.

In addition to these idiosyncratic departures from standard Python semantics, setuptools suffered from being unmaintained. It became a critical part of the Python ecosystem at the same time as the author was moving on to other projects entirely outside of programming. No-one could agree on who the new maintainers should be for a long period of time. The project was forked, and many operating systems’ packaging toolchains calcified around a buggy, ancient version.

From 2008 to 2012 or so, Python packaging was a total mess. It was painful to use. It was not clear which libraries or tools to use, which ones were worth investing in or learning. Doing things the simple way was too tedious, and doing things the automated way involved lots of poorly-documented workarounds and inscrutable failure modes.

This is to say nothing of the fact that there were critical security flaws in various parts of this toolchain. There was no practical way to package and upload Python packages in such a way that users didn’t need a full compiler toolchain for their platform.

To make matters worse for the popular perception of Python’s packaging prowess2, at this same time, newer languages and environments were getting a lot of buzz, ones that had packaging built in at the very beginning and had a much better binary distribution story. These environments learned lessons from the screw-ups of Python and Perl, and really got a lot of things right from the start.

Finally, the Python Package Index, the site which hosts all the open source packages uploaded by the Python community, was basically a proof-of-concept that went live way too early, had almost no operational resources, and was offline all the dang time.

Things were looking pretty bad for Python.


Intermission

Here is where we get to the point of this post - this is where popular opinion about Python packaging is stuck. Outdated information from this period abounds. Blog posts complaining about problems score high in web searches. Those who used Python during this time, but have now moved on to some other language, frequently scoff and dismiss Python as impossible to package, its packaging ecosystem as broken, PyPI as down all the time, and so on. Worst of all, bad advice for workarounds which are no longer necessary are still easy to find, which causes users to pre-emptively break their environments where they really don’t need to.


From The Ashes

In the midst of all this brokenness, there were some who were heroically, quietly, slowly fixing the mess, one gnarly bug-report at a time. pip was started, and its various maintainers fixed much of easy_install’s overcomplexity and many of its flaws. Donald Stufft stepped in both on Pip and PyPI and improved the availability of the systems it depended upon, as well as some pretty serious vulnerabilities in the tool itself. Daniel Holth wrote a PEP for the wheel format, which allows for binary redistribution of libraries. In other words, it lets authors of packages which need a C compiler to build give their users a way to not have one.

In 2013, setuptools and distribute un-forked, providing a path forward for operating system vendors to start updating their installations and allowing users to use something modern.

Python Core started distributing the ensurepip module along with both Python 2.7 and 3.3, allowing any user with a recent Python installed to quickly bootstrap into a sensible Python development environment with a one-liner.

A New Renaissance

I won’t give you a full run-down of the state of the packaging art. There’s already a website for that. I will, however, give you a précis of how much easier it is to get started nowadays. Today, if you want to get a sensible, up-to-date python development environment, without administrative privileges, all you have to do is:

1
2
3
$ python -m ensurepip --user
$ python -m pip install --user --upgrade pip
$ python -m pip install --user --upgrade virtualenv

Then, for each project you want to do, make a new virtualenv:

1
2
3
$ python -m virtualenv lets-go
$ . ./lets-go/bin/activate
(lets-go) $ _

From here on out, now the world is your oyster; you can pip install to your heart’s content, and you probably won’t even need to compile any C for most packages. These instructions don’t depend on Python version, either: as long as it’s up-to-date, the same steps work on Python 2, Python 3, PyPy and even Jython. In fact, often the ensurepip step isn’t even necessary since pip comes preinstalled. Running it if it’s unnecessary is harmless, even!

Other, more advanced packaging operations are much simpler than they used to be, too.

  • Need a C compiler? OS vendors have been working with the open source community to make this easier across the board:
    1
    2
    3
    4
    5
    $ apt install build-essential python-dev # ubuntu
    $ xcode-select --install # macOS
    $ dnf install @development-tools python-devel # fedora
    C:\> REM windows
    C:\> start https://www.microsoft.com/en-us/download/details.aspx?id=44266
    

Okay that last one’s not as obvious as it ought to be but they did at least make it freely available!

  • Want to upload some stuff to PyPI? This should do it for almost any project:

    1
    2
    3
    $ pip install twine
    $ python setup.py sdist bdist_wheel
    $ twine upload dist/*
    
  • Want to build wheels for the wild and wooly world of Linux? There’s an app4 for that.

Importantly, PyPI will almost certainly be online. Not only that, but a new, revamped site will be “launching” any day now3.

Again, this isn’t a comprehensive resource; I just want to give you an idea of what’s possible. But, as a deeply experienced Python expert I used to swear at these tools six times a day for years; the most serious Python packaging issue I’ve had this year to date was fixed by cleaning up my git repo to delete a cache file.

Work Still To Do

While the current situation is good, it’s still not great.

Here are just a few of my desiderata:

  • We still need better and more universally agreed-upon tooling for end-user deployments.
  • Pip should have a GUI frontend so that users can write Python stuff without learning as much command-line arcana.
  • There should be tools that help you write and update a setup.py. Or a setup.python.json or something, so you don’t actually need to write code just to ship some metadata.
  • The error messages that you get when you try to build something that needs a C compiler and it doesn’t work should be clearer and more actionable for users who don’t already know what they mean.
  • PyPI should automatically build wheels for all platforms by default when you upload sdists; this is a huge project, of course, but it would be super awesome default behavior.

I could go on. There are lots of ways that Python packaging could be better.

The Bottom Line

The real takeaway here though, is that although it’s still not perfect, other languages are no longer doing appreciably better. Go is still working through a number of different options regarding dependency management and vendoring, and, like Python extensions that require C dependencies, CGo is sometimes necessary and always a problem. Node has had its own well-publicized problems with their dependency management culture and package manager. Hackage is cool and all but everything takes a literal geological epoch to compile.

As always, I’m sure none of this applies to Rust and Cargo is basically perfect, but that doesn’t matter, because nobody reading this is actually using Rust.

My point is not that packaging in any of these languages is particularly bad. They’re all actually doing pretty well, especially compared to the state of the general programming ecosystem a few years ago; many of them are making regular progress towards user-facing improvements.

My point is that any commentary suggesting they’re meaningfully better than Python at this point is probably just out of date. Working with Python packaging is more or less fine right now. It could be better, but lots of people are working on improving it, and the structural problems that prevented those improvements from being adopted by the community in a timely manner have almost all been addressed.

Go! Make some virtualenvs! Hack some setup.pys! If it’s been a while and your last experience was really miserable, I promise, it’s better now.


Am I wrong? Did I screw up a detail of your favorite language? Did I forget to mention the one language environment that has a completely perfect, flawless packaging story? Do you feel the need to just yell at a stranger on the Internet about picayune details? Feel free to get in touch!


  1. released in October, 2000 

  2. say that five times fast. 

  3. although I’m not sure what it means to “launch” when the site is online, and running against the production data-store, and you can use it for pretty much everything... 

  4. “app” meaning of course “docker container” 

The One Python Library Everyone Needs

Use attrs. Use it. Use it for everything.

Do you write programs in Python? You should be using attrs.

Why, you ask? Don’t ask. Just use it.

Okay, fine. Let me back up.

I love Python; it’s been my primary programming language for 10+ years and despite a number of interesting developments in the interim I have no plans to switch to anything else.

But Python is not without its problems. In some cases it encourages you to do the wrong thing. Particularly, there is a deeply unfortunate proliferation of class inheritance and the God-object anti-pattern in many libraries.

One cause for this might be that Python is a highly accessible language, so less experienced programmers make mistakes that they then have to live with forever.

But I think that perhaps a more significant reason is the fact that Python sometimes punishes you for trying to do the right thing.

The “right thing” in the context of object design is to make lots of small, self-contained classes that do one thing and do it well. For example, if you notice your object is starting to accrue a lot of private methods, perhaps you should be making those “public”1 methods of a private attribute. But if it’s tedious to do that, you probably won’t bother.

Another place you probably should be defining an object is when you have a bag of related data that needs its relationships, invariants, and behavior explained. Python makes it soooo easy to just define a tuple or a list. The first couple of times you type host, port = ... instead of address = ... it doesn’t seem like a big deal, but then soon enough you’re typing [(family, socktype, proto, canonname, sockaddr)] = ... everywhere and your life is filled with regret. That is, if you’re lucky. If you’re not lucky, you’re just maintaining code that does something like values[0][7][4][HOSTNAME][“canonical”] and your life is filled with garden-variety pain rather than the more complex and nuanced emotion of regret.


This raises the question: is it tedious to make a class in Python? Let’s look at a simple data structure: a 3-dimensional cartesian coordinate. It starts off simply enough:

1
class Point3D(object):

So far so good. We’ve got a 3 dimensional point. What next?

1
2
class Point3D(object):
    def __init__(self, x, y, z):

Well, that’s a bit unfortunate. I just want a holder for a little bit of data, and I’ve already had to override a special method from the Python runtime with an internal naming convention? Not too bad, I suppose; all programming is weird symbols after a fashion.

At least I see my attribute names in there, that makes sense.

1
2
3
class Point3D(object):
    def __init__(self, x, y, z):
        self.x

I already said I wanted an x, but now I have to assign it as an attribute...

1
2
3
class Point3D(object):
    def __init__(self, x, y, z):
        self.x = x

... to x? Uh, obviously ...

1
2
3
4
5
class Point3D(object):
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z

... and now I have to do that once for every attribute, so this actually scales poorly? I have to type every attribute name 3 times?!?

Oh well. At least I’m done now.

1
2
3
4
5
6
class Point3D(object):
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z
    def __repr__(self):

Wait what do you mean I’m not done.

1
2
3
4
5
6
7
8
class Point3D(object):
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z
    def __repr__(self):
        return (self.__class__.__name__ +
                ("(x={}, y={}, z={})".format(self.x, self.y, self.z)))

Oh come on. So I have to type every attribute name 5 times, if I want to be able to see what the heck this thing is when I’m debugging, which a tuple would have given me for free?!?!?

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
class Point3D(object):
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z
    def __repr__(self):
        return (self.__class__.__name__ +
                ("(x={}, y={}, z={})".format(self.x, self.y, self.z)))
    def __eq__(self, other):
        if not isinstance(other, self.__class__):
            return NotImplemented
        return (self.x, self.y, self.z) == (other.x, other.y, other.z)

7 times?!?!?!?

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
class Point3D(object):
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z
    def __repr__(self):
        return (self.__class__.__name__ +
                ("(x={}, y={}, z={})".format(self.x, self.y, self.z)))
    def __eq__(self, other):
        if not isinstance(other, self.__class__):
            return NotImplemented
        return (self.x, self.y, self.z) == (other.x, other.y, other.z)
    def __lt__(self, other):
        if not isinstance(other, self.__class__):
            return NotImplemented
        return (self.x, self.y, self.z) < (other.x, other.y, other.z)

9 times?!?!?!?!?

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
from functools import total_ordering
@total_ordering
class Point3D(object):
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z
    def __repr__(self):
        return (self.__class__.__name__ +
                ("(x={}, y={}, z={})".format(self.x, self.y, self.z)))
    def __eq__(self, other):
        if not isinstance(other, self.__class__):
            return NotImplemented
        return (self.x, self.y, self.z) == (other.x, other.y, other.z)
    def __lt__(self, other):
        if not isinstance(other, self.__class__):
            return NotImplemented
        return (self.x, self.y, self.z) < (other.x, other.y, other.z)

Okay, whew - 2 more lines of code isn’t great, but now at least we don’t have to define all the other comparison methods. But now we’re done, right?

1
2
from unittest import TestCase
class Point3DTests(TestCase):

You know what? I’m done. 20 lines of code so far and we don’t even have a class that does anything; the hard part of this problem was supposed to be the quaternion solver, not “make a data structure which can be printed and compared”. I’m all in on piles of undocumented garbage tuples, lists, and dictionaries it is; defining proper data structures well is way too hard in Python.2


namedtuple to the (not really) rescue

The standard library’s answer to this conundrum is namedtuple. While a valiant first draft (it bears many similarities to my own somewhat embarrassing and antiquated entry in this genre) namedtuple is unfortunately unsalvageable. It exports a huge amount of undesirable public functionality which would be a huge compatibility nightmare to maintain, and it doesn’t address half the problems that one runs into. A full enumeration of its shortcomings would be tedious, but a few of the highlights:

  • Its fields are accessable as numbered indexes whether you want them to be or not. Among other things, this means you can’t have private attributes, because they’re exposed via the apparently public __getitem__ interface.
  • It compares equal to a raw tuple of the same values, so it’s easy to get into bizarre type confusion, especially if you’re trying to use it to migrate away from using tuples and lists.
  • It’s a tuple, so it’s always immutable. Sort of.

As to that last point, either you can use it like this:

1
Point3D = namedtuple('Point3D', ['x', 'y', 'z'])

in which case it doesn’t look like a type in your code; simple syntax-analysis tools without special cases won’t recognize it as one. You can’t give it any other behaviors this way, since there’s nowhere to put a method. Not to mention the fact that you had to type the class’s name twice.

Alternately you can use inheritance and do this:

1
2
class Point3D(namedtuple('_Point3DBase', 'x y z'.split()])):
    pass

This gives you a place you can put methods, and a docstring, and generally have it look like a class, which it is... but in return you now have a weird internal name (which, by the way, is what shows up in the repr, not the class’s actual name). However, you’ve also silently made the attributes not listed here mutable, a strange side-effect of adding the class declaration; that is, unless you add __slots__ = 'x y z'.split() to the class body, and then we’re just back to typing every attribute name twice.

And this doesn’t even mention the fact that science has proven that you shouldn’t use inheritance.

So, namedtuple can be an improvement if it’s all you’ve got, but only in some cases, and it has its own weird baggage.


Enter The attr

So here’s where my favorite mandatory Python library comes in.

Let’s re-examine the problem above. How do I make Point3D with attrs?

1
2
import attr
@attr.s

Since this isn’t built into the language, we do have to have 2 lines of boilerplate to get us started: the import and the decorator saying we’re about to use it.

1
2
3
import attr
@attr.s
class Point3D(object):

Look, no inheritance! By using a class decorator, Point3D remains a Plain Old Python Class (albeit with some helpful double-underscore methods tacked on, as we’ll see momentarily).

1
2
3
4
import attr
@attr.s
class Point3D(object):
    x = attr.ib()

It has an attribute called x.

1
2
3
4
5
6
import attr
@attr.s
class Point3D(object):
    x = attr.ib()
    y = attr.ib()
    z = attr.ib()

And one called y and one called z and we’re done.

We’re done? Wait. What about a nice string representation?

1
2
>>> Point3D(1, 2, 3)
Point3D(x=1, y=2, z=3)

Comparison?

1
2
3
4
5
6
>>> Point3D(1, 2, 3) == Point3D(1, 2, 3)
True
>>> Point3D(3, 2, 1) == Point3D(1, 2, 3)
False
>>> Point3D(3, 2, 3) > Point3D(1, 2, 3)
True

Okay sure but what if I want to extract the data defined in explicit attributes in a format appropriate for JSON serialization?

1
2
>>> attr.asdict(Point3D(1, 2, 3))
{'y': 2, 'x': 1, 'z': 3}

Maybe that last one was a little on the nose. But nevertheless, it’s one of many things that becomes easier because attrs lets you declare the fields on your class, along with lots of potentially interesting metadata about them, and then get that metadata back out.

1
2
3
4
5
>>> import pprint
>>> pprint.pprint(attr.fields(Point3D))
(Attribute(name='x', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None),
 Attribute(name='y', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None),
 Attribute(name='z', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None))

I am not going to dive into every interesting feature of attrs here; you can read the documentation for that. Plus, it’s well-maintained, so new goodies show up every so often and I might miss something important. But attrs does a few key things that, once you have them, you realize that Python was sorely missing before:

  1. It lets you define types concisely, as opposed to the normally quite verbose manual def __init__.... Types without typing.
  2. It lets you say what you mean directly with a declaration rather than expressing it in a roundabout imperative recipe. Instead of “I have a type, it’s called MyType, it has a constructor, in the constructor I assign the property ‘A’ to the parameter ‘A’ (and so on)”, you say “I have a type, it’s called MyType, it has an attribute called a”, and behavior is derived from that fact, rather than having to later guess about the fact by reverse engineering it from behavior (for example, running dir on an instance, or looking at self.__class__.__dict__).
  3. It provides useful default behavior, as opposed to Python’s sometimes-useful but often-backwards defaults.
  4. It adds a place for you to put a more rigorous implementation later, while starting out simple.

Let’s explore that last point.

Progressive Enhancement

While I’m not going to talk about every feature, I’d be remiss if I didn’t mention a few of them. As you can see from those mile-long repr()s for Attribute above, there are a number of interesting ones.

For example: you can validate attributes when they are passed into an @attr.s-ified class. Our Point3D, for example, should probably contain numbers. For simplicity’s sake, we could say that that means instances of float, like so:

1
2
3
4
5
6
7
import attr
from attr.validators import instance_of
@attr.s
class Point3D(object):
    x = attr.ib(validator=instance_of(float))
    y = attr.ib(validator=instance_of(float))
    z = attr.ib(validator=instance_of(float))

The fact that we were using attrs means we have a place to put this extra validation: we can just add type information to each attribute as we need it. Some of these facilities let us avoid other common mistakes. For example, this is a popular “spot the bug” Python interview question:

1
2
3
4
5
6
7
class Bag:
    def __init__(self, contents=[]):
        self._contents = contents
    def add(self, something):
        self._contents.append(something)
    def get(self):
        return self._contents[:]

Fixing it, of course, becomes this:

1
2
3
4
5
class Bag:
    def __init__(self, contents=None):
        if contents is None:
            contents = []
        self._contents = contents

adding two extra lines of code.

contents inadvertently becomes a global varible here, making all Bag objects not provided with a different list share the same list. With attrs this instead becomes:

1
2
3
4
5
6
7
@attr.s
class Bag:
    _contents = attr.ib(default=attr.Factory(list))
    def add(self, something):
        self._contents.append(something)
    def get(self):
        return self._contents[:]

There are several other features that attrs provides you with opportunities to make your classes both more convenient and more correct. Another great example? If you want to be strict about extraneous attributes on your objects (or more memory-efficient on CPython), you can just pass slots=True at the class level - e.g. @attr.s(slots=True) - to automatically turn your existing attrs declarations a matching __slots__ attribute. All of these handy features allow you to make better and more powerful use of your attr.ib() declarations.


The Python Of The Future

Some people are excited about eventually being able to program in Python 3 everywhere. What I’m looking forward to is being able to program in Python-with-attrs everywhere. It exerts a subtle, but positive, design influence in all the codebases I’ve seen it used in.

Give it a try: you may find yourself surprised at places where you’ll now use a tidily explained class, where previously you might have used a sparsely-documented tuple, list, or a dict, and endure the occasional confusion from co-maintainers. Now that it’s so easy to have structured types that clearly point in the direction of their purpose (in their __repr__, in their __doc__, or even just in the names of their attributes), you might find you’ll use a lot more of them. Your code will be better for it; I know mine has been.


  1. Scare quotes here because the attributes aren’t meaningfully exposed to the caller, they’re just named publicly. This pattern, getting rid of private methods entirely and having only private attributes, probably deserves its own post... 

  2. And we hadn’t even gotten to the really exciting stuff yet: type validation on construction, default mutable values...