Python Packaging Demystified

IMPORTANT: Some of the information in this post is now obsolete. Please see here.

Randomly take a couple of words from the following list, put them together, and there’s a decent chance you’ll come up with something real involving Python packaging.

  • setup
  • install
  • dist
  • distribute
  • tools
  • utils
  • easy
  • py

The packaging situation is kind of confusing with all the alternate tools, libraries and methods, so I’m going to take a stab at clarifying things — for how it seems right now, anyway.

 What is a package?

Let’s step back for a second to review some Python vocabulary. A single .py file can be a script or a module — the basic difference being that a script is primarily meant to be run, while a module is primarily meant to export useful stuff for scripts or other modules. Of course a module can be executed too, typically with the if __name__ == '__main__' pattern. A package is a group of modules, providing some additional structure. In short, it’s a directory with an __init__.py and some other .py files in it.

 I don’t think that’s what I mean

This is where the vocabulary gets muddled. If you’re familiar with PyPI (pie-pee-eye), the Python Package Index, you’ll realize there’s something more than directories with __init__.py files. PyPI has what are correctly called distributions. Sure, they’re packages (colloquially), but include metadata, documentation, and other stuff to make it easier to manage third-party Python software on your machine.

Getting started

The distribute page cuts through much of the disutils, setuptools, easy_install, soup with this pithy graphic:

http://python-distribute.org/pip_distribute.png

OK, since the New Hotness sounds good, we just want distribute and pip. The graphic even shows you how to get them if you’re on *nix. Observant readers might notice that pip is being installed with easy_install and say, “Hey, wait a minute… easy_install is on the Old and Busted list, but we’re supposed to use that first?”

It turns out that pip (a recursive acronym for “Pip Installs Python”) currently requires distribute, and distribute happens to include easy_install. With Python 3.3 (which, by chance, has gone to release candidate status today), the features of the distribute package become distutils2 in the Python standard library, and distribute is headed for the bit bucket. Sorry, this was supposed to be clarifying things, but it’s getting back into the realm of chaos. So for now, we’ll press forward with distribute as the Not-Totally-New Hotness, but workable strategy for today.

If you’re on Windows, you can just download distribute_setup.py from here:

http://python-distribute.org/distribute_setup.py

Then, as indicated above, you can run this from a command prompt:

python distribute_setup.py

The install creates a Scripts subdirectory in your Python install directory, and puts easy_install.exe in there. It would be a good idea to add the Scripts subdirectory to your Windows PATH. The New Hotness pip.exe goes in there too, once that’s installed. If you go the easy_install route, you’ll get a UAC prompt on:

easy_install pip

Then pip will be installed. Alternatively, (and without the need for UAC elevation) you can download the pip package directly from:

http://pypi.python.org/pypi/pip/

The download is way down at the bottom of the page as a .tar.gz file, which isn’t the most convenient format for Windows, but 7Zip can remedy that difficulty. Once the archive is unpacked, you can install pip with this command from within in the directory where the pip archive was unpacked:

python setup.py install

Once you’ve done this, you may delete the contents of the unpacked archive — everything you need has been copied into your Python install.

Now you can use pip to search for packages on PyPI and install them. For example:

pip search tornado

This will find packages with “tornado” in their name or description, which includes not only the package specifically named “tornado”, but related packages as well. Installing a package is, not surprisingly:

pip install tornado

This downloads the Tornado web server and copies it into the site-packages subdirectory of your Python installation (C:\Python27\Lib\site-packages for example). Since site-packages is in the Python sys.path, you can now import Tornado types from a Python script located anywhere on your system. Uninstalling a package is equally obvious:

pip uninstall tornado

To see what packages you have installed, run:

pip freeze

 Advanced pip

“freeze” seems an odd verb for listing installed packages. This is because there is more to it: You can “freeze” a set of packages with version numbers that are all known to work peacefully together, creating a requirements file that can be used to recreate that same environment somewhere else. More details about this are in the pip documentation.

Let’s say you’re developing a package and want to make that package available to scripts on your system. I guess you could check your source out right into site-packages such that it can be edited in-place, but that seems awkward and unpleasant. Instead, you can install your package with pip using “edit mode”:

pip install -e C:\Projects\Alert\dvsdriver

The path specified here is where my Python project resides from my source control checkout. With the -e option, instead of copying the package to site-packages, a path configuration file (a file with a .pth extension) is created in site-packages instead. The .pth file adds the C:\Projects\Alert\dvsdriver directory to the Python path, allowing you to develop your package in its natural location while still making it available for importing from other scripts.

You can also use pip to install packages from places other than PyPI, including local files, URLs of alternate package indexes, and directly from version control repositories.

Virtual environments

If you get into the situation where you need one version of a package for one script, and different version of the same package for another script, you’ll probably want to look into virtualenv. This is a package that lets you create multiple separate Python environments that are completely isolated, so they can have different sets of packages installed into them. The environments set up with virtualenv already come with distribute and pip in them, so they’re ready to be populated with other packages.

 Creating Distributable Packages

Now that we’ve got a sense of what distributable packages are and how to use them, how do you create one?

The main thing is to create a setup.py module, which looks something like this:

from distutils.core import setup

setup(name='MyProject',
      version='1.0',
      author='Eric Smith',
      packages=['myproject'],
      )

Notice that we’re importing from distutils, which means Old Stuff, but it gives us a working package that can be managed with pip:

pip install -e \Users\Eric\Projects\MyProject

There’s good information about project structure, useful files (like README.txt), additional metadata to include in the setup call, and general package building in The Hitchhiker’s Guide to Packaging.

Looking forward

As mentioned previously, distutils2 is nearly upon us. With that, the setup.py script will be replaced by a declarative setup.cfg and all previous libraries (distutils, distribute, setuptools) will be deprecated. Pip will stick around, though.

So there’s my attempt to clarify Python packaging. If I’ve made egregious errors, please let me know and I’ll fix them.

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