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PYTHONPATH is an environment variable.
See the Python 3 docs for PYTHONPATH.
The PYTHONPATH variable has a value that is a string with a list ofdirectories that Python should add to the sys.path directorylist.
The main use of PYTHONPATH is when we are developing some code that we want tobe able to import from Python, but that we have not yet made into aninstallable Python package (see: making a Python package).
Returning to the example module and script in Where does Python look for modules?:
Contents of code/a_module.py
def func(): print("Running useful function")Contents of scripts/a_script.py
import a_modulea_module.func()
At the moment, on my machine, PYTHONPATH is empty:
$ echo $PYTHONPATH
Before we set PYTHONPATH correctly, a_script.py will fail with:
$ python3 scripts/a_script.pyTraceback (most recent call last): File "scripts/a_script.py", line 1, in <module> import a_moduleModuleNotFoundError: No module named 'a_module'
Now I set the PYTHONPATH environment variable value to be the path to thecode directory:
$ # Set PYTHONPATH to path to the working directory + /code$ # This is for the "bash" shell on Unix / git bash on Windows$ export PYTHONPATH="$PWD/code"$ # Now the script can find "a_module"$ python3 scripts/a_script.pyRunning useful function
Setting PYTHONPATH more permanently¶
You probably don’t want to have to set PYTHONPATH every time you start up aterminal and run a Python script.
Luckily, we can make the PYTHONPATH value be set for any terminal session, bysetting the environment variable default.
For example, let’s say I wanted add the directory /Users/my_user/code tothe PYTHONPATH:
If you are on a Mac¶
Open
Terminal.app;Open the file
~/.bash_profilein your text editor – e.g.atom~/.bash_profile;Add the following line to the end:
export PYTHONPATH="/Users/my_user/code"
Save the file.
Close
Terminal.app;Start
Terminal.appagain, to read in the new settings, and type this:echo $PYTHONPATH
It should show something like
/Users/my_user/code.
If you are on Linux¶
Open your favorite terminal program;
Open the file
~/.bashrcin your text editor – e.g.atom~/.bashrc;Add the following line to the end:
export PYTHONPATH=/home/my_user/code
Save the file.
Close your terminal application;
Start your terminal application again, to read in the new settings, andtype this:
echo $PYTHONPATH
It should show something like
/home/my_user/code.
If you are on Windows¶
Got to the Windows menu, right-click on “Computer” and select “Properties”:
From the computer properties dialog, select “Advanced system settings” on theleft:
From the advanced system settings dialog, choose the “Environment variables”button:
In the Environment variables dialog, click the “New” button in the top half ofthe dialog, to make a new user variable:
Give the variable name as PYTHONPATH and the value is the path tothe code directory. Choose OK and OK again to save thisvariable.
Now open a cmd Window (Windows key, then type cmd and pressReturn). Type:
echo %PYTHONPATH%
to confirm the environment variable is correctly set.
If you want your IPython sessions to see this new PYTHONPATH variable,you’ll have to restart your terminal and restart IPython so that it picks upPYTHONPATH from the environment settings.
Checking system environment variables in Python¶
You can check the current setting of environment variables, using theos.environ dictionary. It contains all the defined environment variablesof the shell that started Python. For example, you can check the value of thePYTHONPATH environment variable, if it is defined:
>>> import os>>> os.environ['PYTHONPATH']'/home/my_user/code'
