What is PYTHONPATH environment variable in Python?

In Python, PYTHONPATH is an environment variable that specifies a list of directories to search for Python modules when importing them. When you import a module in Python, Python looks for the module in the directories specified in sys.path, which is a list of directories that includes the current working directory and directories specified in PYTHONPATH.

PYTHONPATH is an environment variable which you can set to add additional directories where python will look for modules and packages. For most installations, you should not set these variables since they are not needed for Python to run. Python knows where to find its standard library.

The only reason to set PYTHONPATH is to maintain directories of custom Python libraries that you do not want to install in the global default location (i.e., the site-packages directory).

Examples of how to use PYTHONPATH −

Set PYTHONPATH in the terminal

You can set PYTHONPATH in the terminal before running a Python script. Here's an example −

$ export PYTHONPATH=/path/to/my/modules/
$ python my_script.py

Set PYTHONPATH in the script

You can set PYTHONPATH in your Python script using the os module. Here's an example −

import os
import sys
sys.path.insert(0, os.path.abspath("/path/to/my/modules/"))

Append to PYTHONPATH

You can append a directory to PYTHONPATH in your Python script using the os module. Here's an example −

import os
import sys
sys.path.append(os.path.abspath("/path/to/my/modules/"))

Use a .pth file

You can create a .pth file in a directory that contains the directories to be added to PYTHONPATH. Here's an example −

$ cat /path/to/my/modules/my_modules.pth
$/path/to/my/modules/

Use a virtual environment

When you create a virtual environment with venv or virtualenv, it automatically sets PYTHONPATH to include the virtual environment's site-packages directory.Here's an example −

$ python -m venv my_virtualenv
$ source my_virtualenv/bin/activate

Use a .env file

You can use a .env file to set environment variables, including PYTHONPATH, for your project. First, install the python-dotenv package using

pip install python-dotenv

Then, create a .env file in the root directory of your project and add the following line to set PYTHONPATH −

PYTHONPATH=/path/to/my/modules/

Finally, load the environment variables from the .env file in your Python script −

from dotenv import load_dotenv
import os
load_dotenv()
sys.path.insert(0, os.getenv("PYTHONPATH"))

Use a package directory structure

You can create a package directory structure where each subdirectory represents a package, and the subdirectories are automatically added to PYTHONPATH. Here's an example:

import my_package.my_module

And import my_submodule like this:

import my_package.my_subpackage.my_submodule

PYTHONPATH is an environment variable that specifies a list of directories to search for Python modules when importing them. This can be useful when you have custom Python libraries that you do not want to install in the global default location. There are several ways to set PYTHONPATH, including setting it in the terminal, setting it in your Python script using the os module, appending to it using the os module, using a .pth file, using a virtual environment, and using a .env file. By using PYTHONPATH effectively, you can organize your Python code more efficiently and make it easier to maintain.

In conclusion, these are some ways to use PYTHONPATH to specify directories to search for Python modules. The method you choose depends on your needs and preferences, but using a virtual environment or a package directory structure are commonly used methods.