How do I find out my PYTHONPATH using Python?

Key concepts: PYTHONPATH and sys.path

PYTHONPATH is an environment variable that holds a list of directories where Python should look for modules during imports. On launching a Python session, PYTHONPATH directories get attached to a Python list known as sys.path, which can be accessed using the sys module.

Anatomy of sys.path

sys.path is a composite list including:

• Directory containing the input script (or current directory if not specified).
• Directories in PYTHONPATH (if set).
• Installation-dependent default directories.

Remember, sys.path might have extra entries not explicitly set in PYTHONPATH.

Direct access to PYTHONPATH

You can also get the PYTHONPATH in your script through:

import os
pythonpath = os.environ.get('PYTHONPATH', '').split(os.pathsep)
print(pythonpath)
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Note: It returns an empty string if PYTHONPATH isn't set, saving you from pesky errors.

Modifying your Python paths

Augmenting PYTHONPATH from a shell

Expand your PYTHONPATH from a shell using the export command:

export PYTHONPATH="/new/path/to/include:$PYTHONPATH"
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Altering sys.path within your Python script

At runtime, you can change sys.path straight from your scripts:

import sys
sys.path.append("/new/module/path")
# Knock, knock...
# Who's there?
# A new path that Python can search modules in
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Visualizing PYTHONPATH

Consider PYTHONPATH as your GPS for Python modules. Python navigates through the directories from sys.path looking for any module you request.

import sys
print("PYTHONPATH destinations:")
print("\n".join(sys.path))
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Each path functions like a specific address leading to the house of a Python module!

Operating over platforms and Python versions

Cross-platform access to PYTHONPATH

Access and manipulation methods for PYTHONPATH offer universal compatibility, working across operating systems like Windows, macOS, and Linux.

Compatibility with multiple Python versions

Dealing with several Python versions on your system? Make sure you use python2 or python3 to specify the PYTHONPATH for your preferred Python version.

Handling errors and environment

Dealing with an unset PYTHONPATH

If PYTHONPATH isn't set, os.environ will throw a KeyError. To handle this:

import os
try:
    pythonpath = os.environ['PYTHONPATH']
except KeyError:
    print("PYTHONPATH is taking a day off.")
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Caution while modifying sys.path

You can perform list operations on sys.path rendering it dynamic but beware of module resolution conflicts due to reckless changes.

Taking it to the terminal

To get Python paths from the terminal, use this gem:

python -c "import sys; print(sys.path)"
# PYTHONPATH! But this time, straight from the club (terminal), without going through the middleman (Python file)
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This command invokes Python and prints the module path in your terminal.