Gaël Varoquaux

Thu 09 April 2009

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Long sys.path and consequences, one more reason not to use easy_install

For those who don’t know, sys.path is the path that the Python interpreter traverse at each module import to look for the module file imported.

This blog post is about the consequences of having a long sys.path. I’ll try and make it short, but I would have a lot to say. I am just reacting on Noah Gift’s post on performance improvement, not making a full essay on why overloading sys.path is considered harmful.

When using easy_install (or setuptools), each new project is installed in a different directory, and the directory is added at runtime to the sys.path (the addition at runtime confuses many users who are not aware of it). As a result, you quickly end up with more than 40 directory on your sys.path. These directories are ‘stat-ed’ one after the other on each module import. Thus if you have a long sys.path, there are a large amount of system calls to read directories. To check this out, simply try:

strace python -c "import foobar" 2>&1 | less

You can see the amount of noise created by a simple (failing) import statement. On a system with high latency (such as an NFS, as we use at work), this is very costly.

Noah joyfully reports performance improvements by hijacking the Python import mechanism. I claim that part of what Noah has done is not really hijacking the import mechanism, it is undoing the hijacking performed by setuptools.

I know I am being rude, but many people raised this point before, and it is not getting any traction from the setuptools maintainer. I claim that you should not be using setuptools or easy_install if you want performance or control. I claim that you should not be using setuptools unless you understand well what you are doing (which defeats the name easy_install).

The way I install packages when I want good control via easy_install is in a virtual environment to discovered the dependencies, and then:

easy_install -Zeab . package_name

to download the package for each required package, and

python setup.py install --single-version-externally-managed --record ./foobar

if the package itself is using setuptools.

As you can see, setuptools make it really hard to do a clean install. Its a design choice :(.

Another alternative is to use pip which I strongly encourage.

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