Whenever I use sys.path.append, the new directory will be added. However, once I close python, the list will revert to the previous (default?) values. How do I 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.

If you’re using bash (on a Mac or GNU/Linux distro), add this to your ~/.bashrc

export PYTHONPATH="${PYTHONPATH}:/my/other/path"

Instead of manipulating PYTHONPATH you can also create a path configuration file. First find out in which directory Python searches for this information:

python -m site --user-site

For some reason this doesn’t seem to work in Python 2.7. There you can use:

python -c 'import site; site._script()' --user-site

Then create a .pth file in that directory containing the path you want to add (create the directory if it doesn’t exist).

For example:

# find directory
SITEDIR=$(python -m site --user-site)

# create if it doesn't exist
mkdir -p "$SITEDIR"

# create new .pth file with our path
echo "$HOME/foo/bar" > "$SITEDIR/somelib.pth"

This works on Windows

1. On Windows, with Python 2.7 go to the Python setup folder.
2. Open Lib/site-packages.
3. Add an example.pth empty file to this folder.
4. Add the required path to the file, one per each line.

Then you’ll be able to see all modules within those paths from your scripts.

In case anyone is still confused – if you are on a Mac, do the following:

1. Open up Terminal
2. Type open .bash_profile
3. In the text file that pops up, add this line at the end: export PYTHONPATH=$PYTHONPATH:foo/bar
4. Save the file, restart the Terminal, and you’re done

You could add the path via your pythonrc file, which defaults to ~/.pythonrc on linux. ie.

import sys
sys.path.append('/path/to/dir')

You could also set the PYTHONPATH environment variable, in a global rc file, such ~/.profile on mac or linux, or via Control Panel -> System -> Advanced tab -> Environment Variables on windows.

To give a bit more explanation, Python will automatically construct its search paths (as mentioned above and here) using the site.py script (typically located in sys.prefix + lib/python<version>/site-packages as well as lib/site-python). One can obtain the value of sys.prefix:

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

The site.py script then adds a number of directories, dependent upon the platform, such as /usr/{lib,share}/python<version>/dist-packages, /usr/local/lib/python<version>/dist-packages to the search path and also searches these paths for <package>.pth config files which contain specific additional search paths. For example easy-install maintains its collection of installed packages which are added to a system specific file e.g on Ubuntu it’s /usr/local/lib/python2.7/dist-packages/easy-install.pth. On a typical system there are a bunch of these .pth files around which can explain some unexpected paths in sys.path:

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

So one can create a .pth file and put in any of these directories (including the sitedir as mentioned above). This seems to be the way most packages get added to the sys.path as opposed to using the PYTHONPATH.

Note: On OSX there’s a special additional search path added by site.py for ‘framework builds’ (but seems to work for normal command line use of python): /Library/Python/<version>/site-packages (e.g. for Python2.7: /Library/Python/2.7/site-packages/) which is where 3rd party packages are supposed to be installed (see the README in that dir). So one can add a path configuration file in there containing additional search paths e.g. create a file called /Library/Python/2.7/site-packages/pip-usr-local.pth which contains /usr/local/lib/python2.7/site-packages/ and then the system python will add that search path.

For me it worked when I changed the .bash_profile file. Just changing .bashrc file worked only till I restarted the shell.

For python 2.7 it should look like:

export PYTHONPATH="$PYTHONPATH:/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python"

at the end of the .bash_profile file.

On linux you can create a symbolic link from your package to a directory of the PYTHONPATH without having to deal with the environment variables. Something like:

ln -s /your/path /usr/lib/pymodules/python2.7/

Adding export PYTHONPATH="${PYTHONPATH}:/my/other/path" to the ~/.bashrc might not work if PYTHONPATH does not currently exist (because of the :).

export PYTHONPATH="/my/other/path1"
export PYTHONPATH="${PYTHONPATH}:/my/other/path2"

Adding the above to my ~/.bashrc did the trick for me on Ubuntu 16.04

On MacOS, Instead of giving path to a specific library. Giving full path to the root project folder in

~/.bash_profile 

made my day, for example:

export PYTHONPATH="${PYTHONPATH}:/Users/<myuser>/project_root_folder_path"

after this do:

source ~/.bash_profile

Just to add on awesomo’s answer, you can also add that line into your ~/.bash_profile or ~/.profile

The add a new path to PYTHONPATH is doing in manually by:

adding the path to your ~/.bashrc profile, in terminal by:

vim ~/.bashrc

paste the following to your profile

export PYTHONPATH="${PYTHONPATH}:/User/johndoe/pythonModule"

then, make sure to source your bashrc profile when ever you run your code in terminal:

source ~/.bashrc 

Hope this helps.

I added permanently in Windows Vista, Python 3.5

System > Control Panel > Advanced system settings > Advanced (tap) Environment Variables > System variables > (if you don’t see PYTHONPATH in Variable column) (click) New > Variable name: PYTHONPATH > Variable value:

Please, write the directory in the Variable value. It is details of Blue Peppers’ answer.

The script below works on all platforms as it’s pure Python. It makes use of the pathlib Path, documented here https://docs.python.org/3/library/pathlib.html, to make it work cross-platform. You run it once, restart the kernel and that’s it. Inspired by https://medium.com/@arnaud.bertrand/modifying-python-s-search-path-with-pth-files-2a41a4143574.

from pathlib import Path
to_add=Path(path_of_directory_to_add)
from sys import path

if str(to_add) not in path:
    minLen=999999
    for index,directory in enumerate(path):
        if 'site-packages' in directory and len(directory)<=minLen:
            minLen=len(directory)
            stpi=index

    pathSitePckgs=Path(path[stpi])
    with open(str(pathSitePckgs/'current_machine_paths.pth'),'w') as pth_file:
        pth_file.write(str(to_add))

This is an update to this thread which has some old answers.

For those using MAC-OS Catalina or some newer (>= 10.15), it was introduced a new Terminal named zsh (a substitute to the old bash).

I had some problems with the answers above due to this change, and I somewhat did a workaround by creating the file ~/.zshrc and pasting the file directory to the $PATH and $PYTHONPATH

So, first I did:

nano ~/.zshrc

When the editor opened I pasted the following content:

export PATH="${PATH}:/Users/caio.hc.oliveira/Library/Python/3.7/bin"
export PYTHONPATH="${PYTHONPATH}:/Users/caio.hc.oliveira/Library/Python/3.7/bin"

saved it, and restarted the terminal.

IMPORTANT: The path above is set to my computer’s path, you would have to adapt it to your python.

In Python 3.6.4 you can persist sys.path across python sessions like this:

import sys
import os

print(str(sys.path))

dir_path = os.path.dirname(os.path.realpath(__file__))
print(f"current working dir: {dir_path}")

root_dir = dir_path.replace("/util", '', 1)
print(f"root dir: {root_dir}")

sys.path.insert(0, root_dir)

print(str(sys.path))

I strongly suggest you use virtualenv and virtualenvwrapper otherwise you will clutter your path

Shortest path between A <-> B is a straight line;

import sys
if not 'NEW_PATH' in sys.path:
  sys.path += ['NEW_PATH']