PYTHONPATH vs sys path

Question

Another developer and I disagree about whether PYTHONPATH or sys path should be used to allow Python to find a Python package in a user  e g   development  directory   We have a Python project with a typical directory structure   Project     setup py     package           init   py         lib py         script py   In script py  we need to do import package lib   When the package is installed in site-packages  script py can find package lib   When working from a user directory  however  something else needs to be done   My solution is to set my PYTHONPATH to include    Project    Another developer wants to put this line of code in the beginning of script py   sys path append os path dirname os path dirname os path abspath   file         So that Python can find the local copy of package lib     I think this is a bad idea  as this line is only useful for developers or people running from a local copy  but I can t give a good reason why it is a bad idea   Should we use PYTOHNPATH  sys path  or is either fine

User · Accepted Answer

If the only reason to modify the path is for developers working from their working tree  then you should use an installation tool to set up your environment for you   virtualenv is very popular  and if you are using setuptools  you can simply run setup py develop to semi-install the working tree in your current Python installation

User · Answer

Neither hacking PYTHONPATH nor sys path is a good idea due to the before mentioned reasons  And for linking the current project into the site-packages folder there is actually a better way than python setup py develop  as explained here   pip install --editable path to project  If you don t already have a setup py in your project s root folder  this one is good enough to start with   from setuptools import setup setup  project

User · Answer

Along with the many other reasons mentioned already  you could also point outh that hard-coding    sys path append os path dirname os path dirname os path abspath   file        is brittle because it presumes the location of script py -- it will only work if script py is located in Project package  It will break if a user decides to move copy symlink script py  almost  anywhere else

User · Answer

I think  that in this case using PYTHONPATH is a better thing  mostly because it doesn t introduce  questionable  unneccessary code    After all  if you think of it  your user doesn t need that sys path thing  because your package will get installed into site-packages  because you will be using a packaging system    If the user chooses to run from a  local copy   as you call it  then I ve observed  that the usual practice is to state  that the package needs to be added to PYTHONPATH manually  if used outside the site-packages

User · Answer

I hate PYTHONPATH  I find it brittle and annoying to set on a per-user basis  especially for daemon users  and keep track of as project folders move around  I would much rather set sys path in the invoke scripts for standalone projects   However sys path append isn t the way to do it  You can easily get duplicates  and it doesn t sort out  pth files  Better  and more readable   site addsitedir   And script py wouldn t normally be the more appropriate place to do it  as it s inside the package you want to make available on the path  Library modules should certainly not be touching sys path themselves  Instead  you d normally have a hashbanged-script outside the package that you use to instantiate and run the app  and it s in this trivial wrapper script you d put deployment details like sys path-frobbing

User · Answer

In general I would consider setting up of an environment variable  like PYTHONPATH   to be a bad practice  While this might be fine for a one off debugging but using this as a regular practice might not be a good idea     Usage of environment variable leads to  situations like  it works for me  when some one else reports problems in the code base  Also one might carry the same practice with the test environment as well  leading to situations like the tests running fine for a  particular developer but probably failing when some one launches the tests

[python] PYTHONPATH vs. sys.path

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