how to set different PYTHONPATH variables for python3 and python2 respectively

Question

I want to add a specific library path only to python2. After adding export PYTHONPATH="/path/to/lib/" to my .bashrc, however, executing python3 gets the error: Your PYTHONPATH points to a site-packages dir for Python 2.x but you are running Python 3.x!

I think it is due to that python2 and python3 share the common PYTHONPATH variable.

So, can I set different PYTHONPATH variables respectively for python2 and python3. If not, how can I add a library path exclusively to a particular version of python?

Answer 1

PYTHONPATH is somewhat of a hack as far as package management is concerned. A "pretty" solution would be to package your library and install it.

This could sound more tricky than it is, so let me show you how it works.

Let us assume your "package" has a single file named wow.py and you keep it in /home/user/mylib/wow.py.

Create the file /home/user/mylib/setup.py with the following content:

from setuptools import setup

setup(name="WowPackage",
      packages=["."],
)

That's it, now you can "properly install" your package into the Python distribution of your choice without the need to bother about PYTHONPATH. As far as "proper installation" is concerned, you have at least three options:

• "Really proper". Will copy your code to your python site-packages directory:

  $ python setup.py install
  

• "Development". Will only add a link from the python site-packages to /home/user/mylib. This means that changes to code in your directory will have effect.

  $ python setup.py develop
  

• "User". If you do not want to write to the system directories, you can install the package (either "properly" or "in development mode") to /home/user/.local directory, where Python will also find them on its own. For that, just add --user to the command.

  $ python setup.py install --user
  $ python setup.py develop --user
  

To remove a package installed in development mode, do

$ python setup.py develop -u

or

$ python setup.py develop -u --user

To remove a package installed "properly", do

 $ pip uninstall WowPackage

If your package is more interesting than a single file (e.g. you have subdirectories and such), just list those in the packages parameter of the setup function (you will need to list everything recursively, hence you'll use a helper function for larger libraries). Once you get a hang of it, make sure to read a more detailed manual as well.

In the end, go and contribute your package to PyPI -- it is as simple as calling python setup.py sdist register upload (you'll need a PyPI username, though).