In Python Script, How to Set Pythonpath

In Python script, how do I set PYTHONPATH?

You don't set PYTHONPATH, you add entries to sys.path. It's a list of directories that should be searched for Python packages, so you can just append your directories to that list.

sys.path.append('/path/to/whatever')

In fact, sys.path is initialized by splitting the value of PYTHONPATH on the path separator character (: on Linux-like systems, ; on Windows).

You can also add directories using site.addsitedir, and that method will also take into account .pth files existing within the directories you pass. (That would not be the case with directories you specify in PYTHONPATH.)

Add folder to PYTHONPATH programmatically

If you only want to change it for the execution of the current script, you can do it simply by assigning (or changing an existing) value in the os.environ mapping. The code below is complicated a bit by the fact that I made it work even if os.environ[PYTHONPATH] isn't initially set to anything (as is the case on my own system).

import os
from pathlib import Path

PYTHONPATH = 'PYTHONPATH'

try:
    pythonpath = os.environ[PYTHONPATH]
except KeyError:
    pythonpath = ''

print('BEFORE:', pythonpath)

folder = Path(__file__).resolve().parent.joinpath('code')
print(f'{folder=}')

pathlist = [str(folder)]
if pythonpath:
    pathlist.extend(pythonpath.split(os.pathsep))
print(f'{pathlist=}')

os.environ[PYTHONPATH] = os.pathsep.join(pathlist)
print('AFTER:', os.environ[PYTHONPATH])

set pythonpath before import statements

This will add a path to your Python process / instance (i.e. the running executable). The path will not be modified for any other Python processes. Another running Python program will not have its path modified, and if you exit your program and run again the path will not include what you added before. What are you are doing is generally correct.

set.py:

import sys
sys.path.append("/tmp/TEST")

loop.py

import sys
import time
while True:
  print sys.path
  time.sleep(1)

run: python loop.py &

This will run loop.py, connected to your STDOUT, and it will continue to run in the background. You can then run python set.py. Each has a different set of environment variables. Observe that the output from loop.py does not change because set.py does not change loop.py's environment.

A note on importing

Python imports are dynamic, like the rest of the language. There is no static linking going on. The import is an executable line, just like sys.path.append....

How to correctly set PYTHONPATH for Visual Studio Code

OP seemed to have asked about path syntax for the .env file and the vscode set up so that it finds and reads some custom module files. My problem was similar in that I wanted code to find my custom modules for import in a script. I did not want to put my custom modules in a folder inside my python environment. I also wanted to avoid setting one or more paths as PYTHONPATH for the User Variables in the Windows Environment Variables - but this will work if you want to do it.
I am working in vscode in Windows 10.

1) SYNTAX:

a) I found that the following path syntax works in the env file:

PYTHONPATH = C:/0APPS/PYTHON/_MODULES

My .py module files are in this folder.

b) # works for comments in the .env file.

2) VSCODE SET-UP: I found that the following works:

a) Like sunew said at #2 My setup: Use the Explorer in vscode to open at your selected project workspace folder. For me that is Q:\420 PYTHON

b) Give the env file a name, like vscode.env file and place it in that folder at the top level of the workspace.

c) Open vscode settings and search .env where under the Extensions > Python you will find "Python: env file". Edit the box to add your env file name just before .env.
e.g. ${workspaceFolder}/vscode.env

d) import custom_modulename now work for me - in the python interactive window and in a script.

Python - add PYTHONPATH during command line module run

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

Permanently add a directory to PYTHONPATH?

You need to add your new directory to the environment variable PYTHONPATH, separated by a colon from previous contents thereof. In any form of Unix, you can do that in a startup script appropriate to whatever shell you're using (.profile or whatever, depending on your favorite shell) with a command which, again, depends on the shell in question; in Windows, you can do it through the system GUI for the purpose.

superuser.com may be a better place to ask further, i.e. for more details if you need specifics about how to enrich an environment variable in your chosen platform and shell, since it's not really a programming question per se.

Bash script which sets pythonpath

Wonder if this is still relevant for you, but I've marked it as fun puzzle to revisit later... Long story short: It adds directory of location where this script file is with all symbolic links resolved (neither the acquired filename nor a directory leading up to is is a symbolic link) to PYTHONPATH.

It's basically the same thing as doing so using readlink (or realpath):

export PYTHONPATH="$(dirname $(readlink -f ${BASH_SOURCE})):${PYTHONOATH}"

Line by line dissection:

SOURCE="${BASH_SOURCE[0]}"

This sets SOURCE to be path with which this script was called or sourced.

while [ -h "$SOURCE" ]; do # resolve $SOURCE until the file is no longer a symlink

We enter the wile loop if SOURCE path refers to a symbolic link. I.e. in the first iteration of this file was a symbolic link. Subsequently if this was a link pointing to another link.

  DIR="$( cd -P "$( dirname "$SOURCE" )" && pwd )"

This (a bit simplified explanation of -P) changes into directory where SOURCE is resolving symbolic links along the way (i.e. lands in the directory the link(s) was/were pointing to) and prints working directory after that change (absolute path). All that happens in a subshell and result is assign to variable DIR.

  SOURCE="$(readlink "$SOURCE")"

SOURCE is assigned a new value of path resulting from symlink resolution. Literally a target the link points to (as seen by for instance ls -l) relative or absolute.

  [[ $SOURCE != \/* ]] && SOURCE="$DIR/$SOURCE" # if $SOURCE was a relative symlink, we need to resolve it relative to the path where the symlink file was located

If the SOURCE value we have obtained by symbolic link resolution does not begin with / (i.e. is an absolute path), DIR (directory where the SOURCE with which we have entered the loop resides) and resolved symbolic link SOURCE are concatenated over / to form a new SOURCE (we make it into an absolute path) and we go back to the top of this loop. NOTE: escaping of / by \ seems in this case unnecessary and arbitrary.

done

When done. SOURCE points to a file that is not a symblic link. It's path may still contain symbolic links at this point which is taken care of in the next step.

DIR="$( cd -P "$( dirname "$SOURCE" )" && pwd )"

Once more, like in the loop. DIR should now be pointing to a directory where resolved (not a symlink) SOURCE file (target of what was originally called/sourced) resides.

# Set the python io encoding to UTF-8 by default if not set.
if [ -z ${PYTHONIOENCODING+x} ]; then export PYTHONIOENCODING=utf8; fi

Exports an environmental variable if a shell variable was not set or equals to an empty string. NOTE: ${PYTHONIOENCODING+x} seems to be an alternative form of ${PYTHONIOENCODING:+x} and its use seems absolutely arbitrary. There is also a test to check if variable was set (regardless of its value).

export PYTHONPATH="${DIR}:${PYTHONPATH}"

PYTHONPATH is now set to start with an absolute resolved path (no symbolic links should be anywhere along the path) of where does this very script (or file this link points to) reside.

python -m mssqlcli.main "$@"

Calls python...