Does the python sys.path act like Unix PATH?

Question

Let's say I have a local development directory with modules that I'm writing, and a system-wide site-packages directory where these modules will eventually be installed. In the case where I'm modifying an existing module and I want to test it I would like to be sure that the module I'm importing is the one in my local site-packages directory. Reading about the site module it seems like I can have a file with a .pth suffix where each line in that file will be appended to the sys.path variable. I've set that up, and I can confirm that my development directory is showing up later in the list, but when I import a module it's importing from the system wide directory.

Simplified Example:

$ virtualenv test_sys
$ cd test_sys
$ source bin/activate
$ mkdir site_modules
$ mkdir user_modules

In each site_modules and user_modules I've put a directory called test_module with the following in the __init__.py:

import os

print(os.path.abspath(__file__))

Now I've added a file called site_modules.pth to test_sys/lib/python2.7/site-packages:

/home/chris/test_sys/site_modules
/home/chris/test_sys/user_modules

And I expect that when I import test_module from the python interpreter it would print the user_modules directory but it doesn't:

>>> import test_module
/home/chris/test_sys/site_modules/test_module/__init__.pyc
>>>

Yes, I've ensured that user_modules shows up later in sys.path than site_modules.

How can I ensure that a module that exists in multiple directories in sys.path will always be imported from a specific directory?

Also see The Module Search Path in the Python docs.

Lukas Graf
– Lukas Graf

2013-11-18 18:42:32 +00:00
Commented Nov 18, 2013 at 18:42 — Lukas Graf
– Lukas Graf, Commented Nov 18, 2013 at 18:42

Dietrich Epp · Accepted Answer · 2013-11-18 18:21:31Z

1

Python's sys.path does work mostly like the Unix PATH environment variable. Both of them use the first object they find in the path. So if you want to have user_modules override site_modules, you have to put it first, not second:

/home/chris/test_sys/user_modules
/home/chris/test_sys/site_modules

This is the same as Unix PATH

PATH=/usr/local/bin:/usr/bin:/bin

Which means that local packages (ones not installed by the package manager) from /usr/local will override system packages in /usr.

answered Nov 18, 2013 at 18:21

Dietrich Epp

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