Understanding .pth in site-packages

OKB (not okblacke) brenNOSPAMbarn at NObrenSPAMbarn.net
Sun Aug 28 07:20:23 CEST 2011

Josh English wrote:

> OKB,
> The setup.py script created the egg, but not the .pth file. I
> created that myself. 
> Thank you for clarifying about how .pth works. I know "redirect
> imports" was the wrong phrase, but it worked in my head at the
> time. It appears, at least on my system, that Python will find
> site-packages/foo before it finds and reads site-packages/foo.pth. 
> At least this solution gives me a way to develop my libraries
> outside of site-packages. 

    	Well, I'm still not totally sure what your setup is, but assuming 
site-packages/foo is a directory containing an __init__.py (that is, it 
is a package), then yes, it will be found before an alternative package 
in a directory named with a .pth file.  Note that I don't say it will be 
found before the .pth file, because, again, the finding of the package 
(when you do "import foo") happens much later than the processing of the 
.pth file.  So it doesn't find site-packages/foo before it reads 
foo.pth; it just finds site-packages/foo before it finds the other foo 
that foo.pth was trying to point to.

    	Let's say your .pth file specifies the directory /elsewhere.  The 
.pth file is processed by site.py when the interpreter starts up, and at 
that time /elsewhere  will be appended to sys.path.  Later, when you do 
the import, it searches sys.path in order.  site-packages itself will be 
earlier in sys.path than /elsewhere, so a package site-packages/foo will 
be found before /elsewhere/foo.  The key here is that the .pth file is 
processed at interpreter-start time, but the search for foo doesn't take 
place until you actually execute "import foo".

    	If you want to make your /elsewhere "jump the line" and go to the 
front, look at easy_install.pth, which seems to have some magic code at 
the end that moves its eggs ahead of site-packages in sys.path.  I'm not 
sure how this works, though, and it seems like a risky proposition.

--OKB (not okblacke)
Brendan Barnwell
"Do not follow where the path may lead.  Go, instead, where there is
no path, and leave a trail."
	--author unknown

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