The environment variable PYTHONPATH is actually only added to the list of locations Python searches for modules. You can print out the full list in the terminal like this:

python -c "import sys; print(sys.path)"

Or if want the output in the UNIX directory list style (separated by :) you can do this:

python -c "import sys; print(':'.join(x for x in sys.path if x))"

Which will output something like this:

/usr/local/lib/python2.7/dist-packages/feedparser-5.1.3-py2.7.egg:/usr/local/lib/
python2.7/dist-packages/stripogram-1.5-py2.7.egg:/home/qiime/lib:/home/debian:/us
r/lib/python2.7:/usr/lib/python2.7/plat-linux2:/usr/lib/python2.7/lib-tk:/usr/lib
/python2.7/lib-old:/usr/lib/python2.7/lib- dynload:/usr/local/lib/python2.7/dist-
packages:/usr/lib/python2.7/dist-packages:/usr/lib/python2.7/dist-packages/PIL:/u
sr/lib/python2.7/dist-packages/gst-0.10:/usr/lib/python2.7/dist-packages/gtk-2.0:
/usr/lib/pymodules/python2.7

PATH variable defines the directories which are searched when executing commands. However when you execute python sleep_plotter.py 113testCtM113.txt, sleep_plotter.py is an argument to the python program (command). Shell uses PATH to find python, but not its arguments.

You can add an executable attribute to your script:

$ chmod +x /Users/myname/python_functions/bin/sleep_plotter.py

Add a shebang sequence to the top (first line) of your Python script:

#!/usr/bin/env python

And run the script directly as a command:

$ sleep_plotter.py 113testCtM113.txt

The variable PYTHONPATH that you echo in the terminal is added to the other paths of python. So if you don't have any particular path set in your .profile or .bashrc file (or locally), the variable will be empty.

To see the path that python uses do in a python shell

import sys
print(sys.path)

Or as @mgilson suggestes, you can run from terminal

python -c 'import sys; print(sys.path)'

A note: If you decide to install by hand a package using python setup.py install --user you don't need to add $HOME/.local/lib/pythonX.X/site-packages to PYTHONPATH, as it is already in sys.path

I'm running Mint 19.3 and kernel version 4.15.0-106-generic - so not far off from you.

(python3-venv) me@mycomp:chap10-Batteries_Included$ cat/etc/*release
DISTRIB_ID=LinuxMint
DISTRIB_RELEASE=19.3
DISTRIB_CODENAME=tricia
DISTRIB_DESCRIPTION="Linux Mint 19.3 Tricia"
...
(python3-venv) me@mycomp:chap10-Batteries_Included$ uname -r
4.15.0-106-generic

running in a Python 3 virtual environment. I'm working on "Beginning Python, From Novice to Professional" by Magnus Lie Hetland and got to Chapter 10 where it talks about this very thing. When I first get into my shell, I find nothing assigned to $PYTHONPATH either:

(python3-venv) me@mycomp:chap10-Batteries_Included$ echo $PYTHONPATH
 
(python3-venv) me@mycomp:chap10-Batteries_Included$

So I do what is suggested everywhere on the web, and in the book($PWD being my working directory, which happened to be the directory I wanted added):

(python3-venv) me@mycomp:chap10-Batteries_Included$ export PYTHONPATH=$PYTHONPATH:$PWD
(python3-venv) me@mycomp:chap10-Batteries_Included$ echo $PYTHONPATH
:/home/me/Documents/CodingProjects/python/Beginning-Python/book/chap10-Batteries_Included
(python3-venv) me@mycomp:chap10-Batteries_Included$

And there we go, it's there:

(python3-venv) me@mycomp:chap10-Batteries_Included$ python -O
Python 3.6.9 (default, Apr 18 2020, 01:56:04) 
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys, pprint
>>> pprint.pprint(sys.path)
['',
 '/home/me/Documents/CodingProjects/python/Beginning-Python/book/chap10-Batteries_Included',
 '/usr/lib/python36.zip',
 '/usr/lib/python3.6',
 '/usr/lib/python3.6/lib-dynload',
 '/home/me/python3-venv/lib/python3.6/site-packages']
>>> 

I noticed, and would have commented had I been able to without having 50 points on here, that you have export export PYTHONPATH=$PYTHONPATH:.... - you only need export once. You can add this export PYTHONPATH=... to your ~/.profile to make it permanent, too. I hope this helps.

Check the documentation again:

It says

[sys.path is initialized] from the environment variable PYTHONPATH, plus an installation-dependent default.

And furthermore,

the first item of this list, path[0], is the directory containing the script that was used to invoke the Python interpreter [… or] the empty string

That’s why yours isn’t empty.

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

Typically, the environment variable $PYTHONPATH is empty (try echo $PYTHONPATH). The actual list of folders python searches for libraries can be found with (in python):

import sys
print(sys.path)

This will consist (in search order) of the current directory, any directories in your $PYTHONPATH, and finally the default library directories, set by site.py. The main default locations are (where X.Y is the python version, eg 2.7):

/usr/lib/pythonX.Y (python system libraries, eg re, urllib)
/usr/lib/pythonX.Y/dist-packages (python libraries installed with deb packages)

If you use pip install --user or similar to install libraries as yourself, it will also contain the user library directory:

/home/USERNAME/.local/lib/pythonX.Y/site-packages

You can manipulate the PYTHONPATH by either setting the environment variable before you launch python (PYTHONPATH=$PYTHONPATH:/foo/bar), or by editing sys.path once you've started python (import sys; sys.path = ["/foo/bar"] + sys.path).

However, if you want to play with your own python libraries, a good idea is to create a virtualenv. This is a directory in which you can play around with your own versions of python libraries without any risk of messing up the python libraries used by the system. See How to set up and use a virtual python environment in Ubuntu? for information about creating and using a virtualenv.

You would probably also want this:

import sys
print(sys.path)

Or as a one liner from the terminal:

python -c "import sys; print('\n'.join(sys.path))"

Caveat: If you have multiple versions of Python installed you should use a corresponding command python2 or python3.

You can type which python on the ubuntu terminal and it will give the Python installed location path.

Module search path:

• First it searches for a built-in module

• Next it searches for the module in a list of directories given by sys.path, which is initialized from these locations:

  • The directory containing the input script (or current directory)
  • PYTHONPATH (a list of directory names, with the same syntax as the shell variable PATH)
  • The installation-dependent default

How to alter sys.path:

• sys.path.insert(_number_, '/whateverfilepath/')
• Use 1 instead of 0 for number so that Python will look in the current working directory first

You can't use a list in fish for PYTHONPATH, you have to use a single string with : as a separator as you would do in bash:

set -gx PYTHONPATH "/opt/additionalPath:/usr/local/lib/python2.7/site-packages"

See this bug report for some more info.

I suggest not to rely on the raw PYTHONPATH because it can vary depending on the OS.

Instead of the PYTHONPATH value in the os.environ dict, use sys.path from the sys module. This is preferrrable, because it is platform independent:

import sys
print(sys.path)

The value of sys.path is initialized from the environment variable PYTHONPATH, plus an installation-dependent default (depending on your OS). For more info see

https://docs.python.org/2/library/sys.html#sys.path

https://docs.python.org/3/library/sys.html#sys.path

I don't have a windows computer, but you could probably use os.environ to get it without relying on an external utility -- On *NIX it looks something like:

import os
print (os.environ['PATH'])

You know what has worked for me really well on windows.

My Computer > Properties > Advanced System Settings > Environment Variables >

Just add the path as C:\Python27 (or wherever you installed python)

OR

Then under system variables I create a new Variable called PythonPath. In this variable I have C:\Python27\Lib;C:\Python27\DLLs;C:\Python27\Lib\lib-tk;C:\other-folders-on-the-path

This is the best way that has worked for me which I hadn't found in any of the docs offered.

EDIT: For those who are not able to get it, Please add

C:\Python27;

along with it. Else it will never work.

It's not clear what your problem could be, but note that you don't need to add the default value of sys.path to your PYTHONPATH variable. The directories you put in PYTHONPATH are additional directories to search; the system default is appended to your PYTHONPATH. In other words, roughly speaking:

sys.path = ":".split( os.environ['PYTHONPATH'] ) + sys.path

Showing the exact value of PYTHONPATH and the resultant errors would help us determine the problem.

For Mac/Linux;

PYTHONPATH=/foo/bar/baz python somescript.py somecommand

For Windows, setup a wrapper pythonpath.bat;

@ECHO OFF
setlocal
set PYTHONPATH=%1
python %2 %3
endlocal

and call pythonpath.bat script file like;

pythonpath.bat /foo/bar/baz somescript.py somecommand