$PYTHONPATH and sys.path don't seems to match

Question

I want to add a folder permanently to sys.path but it doesn't work.

On my Debian Stretch, when I execute :

$ echo $PYTHONPATH

it looks empty

In the same time, when I print the content of sys.path, there is 10 different paths which aren't the same than those contained into $PATH (and obviously in $PYTHONPATH). How can it be explained ?

(I had to create a virtualenv for a framework but I don't think I'm using it right now so it's probably not related but I'm not confortable with that concept so maybe I miss something)

Also I've tried to modify .bashrc to add a path to $PYTHONPATH so I've know the following line :

PYTHONPATH=$PYTHONPATH:~/bin 

After rebooting, it works, I mean $PYTHONPATH contains it but nothing change for sys.path.

Also, $ env | grep ^PYTHON returns nothing.

So, how sys.path is filled and how can I add permanently a folder to it ?

Solution

sys.path in Python is a "list of strings that specifies the search path for modules. Initialized from the environment variable PYTHONPATH, plus an installation-dependent default." (taken from the Python online documentation).

For sys.path to be affected by PYTHONPATH, the latter has to be an environment variable. If it's not an environment variable but just a shell variable, it will not be visible by the python interpreter. You turn it into an environment variable by exporting it after setting its value, or at the same time, as in

export PYTHONPATH=/some/path:/goes/here

Comments

[snoob dogg]

I know that there aren't related but, in the same time, I can't figure out where that list of folder comes from. I thought I was clear about the issue, I can't add a folder to sys.path by modifying $PYTHONPATH.

[Kusalananda]

Did you export PYTHONPATH?

[snoob dogg]

By the way, since my PYTHONPATH was empty of have you an idea, how does the other folders paths were determined ?

[Kusalananda]

@snoobdogg It's dependent on the installation of Python. For a slightly longer explanation, see here: stackoverflow.com/a/38403654/931027