Monday, December 7, 2009

Python package installations

This post is really for me, in the sense of "letting google organize my head." I'm getting into PyCogent (first post here). They're switching over to using matplotlib, which has complicated installation instructions for OS X. This has always scared me away before. But, I want PyCogent to work. Before I get into that, I need to re-learn what Python installation tools are about.

I'm sure any Pythonista knows all this already, but here goes...

What is easy_install?

Easy Install comes from PEAK (Python Enterprise Application Kit). The docs are here. How did it get on my Snow Leopard system? According to the PyCogent docs front page, it comes with the stock Python install on Snow Leopard. I couldn't find it by searching the system. My site-packages directory /Library/Python/2.6/site-packages has a file named easy-install.pth but that's it. easy_install must be somewhere because this works:

easy_install --help

In particular the output of that command includes:

--upgrade (-U)
force upgrade (searches PyPI for latest versions)

usage: easy_install [options] requirement_or_url ...

According to the docs easy_install is related to setuptools (see below), like a "wrapper" for them. According to the Pylons docs, it is actually part of the setuptools module. What about that easy-install.pth file?

According to the same page:
Easy Install works by maintaining a file in your Python installation’s lib/site-packages directory called easy_install.pth. Python looks in .pth files when it is starting up and adds any module ZIP files or paths specified to its internal sys.path. Easy Install simply maintains the easy_install.pth file so that Python can find the packages you have installed with Easy Install.

The first four lines of my easy-install.pth are:

import sys; sys.__plen = len(sys.path)

Now, I think numpy-1.3 is stock on Snow Leopard. The time stamp on the referenced file is Friday, September 4, 2009 8:38 AM. I think that is the date of my upgrade to Snow Leopard, so it checks. easy_install probably comes with Snow Leopard and is used to do the numpy install.

What is setuptools?

Python code to "build, install, upgrade and uninstall Python package - easily!" It comes from here. It is what we're using when we do:

python build
sudo python install

So what is PyPi? It's the Python Package Index, "a repository of software for the Python programming language…currently 8369 packages." It either is or used to be named CheeseShop (a Monty Python thing). According to this tutorial, easy_install "gives you a quick and painless way to install packages remotely." Ah... now I get it.

What is pip?

The PyCogent Quick Installation instructions mention this. According to PyPi pip is intended as a replacement for easy_install. Among other things, pip has explicit requirement files. For example, cogent-requirements.txt:


Note that Apple has provided a special place for /site-packages to live. It is not where you might expect:

(although there is a lot there) but rather here:
You'll can see this in sys.path.

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