which python vs PYTHONPATH

Question

If I type in which python I get: /home/USER/anaconda3/bin/python If I type in echo $PYTHONPATH I get: /home/USER/terrain_planning/devel/lib/python2.7/dist-packages:/opt/ros/melodic/lib/python2.7/dist-packages

Should that not be the same? And is it not better to set it: usr/lib/python/ How would I do that? Add it to the PYTHONPATH or set the PYTHONPATH to that? But how to set which python?

Answer 1

You're mixing 2 environment variables:

• PATH where which looks up for executables when they're accessed by name only. This variable is a list (colon/semi-colon separated depending on the platform) of directories containing executables. Not python specific. which python just looks in this variable and prints the full path
• PYTHONPATH is python-specific list of directories (colon/semi-colon separated like PATH) where python looks for packages that aren't installed directly in the python distribution. The name & format is very close to system/shell PATH variable on purpose, but it's not used by the operating system at all, just by python.

Answer 2

which python is the path to your python interpreter. PYTHONPATH is an environment variable where your Python program can search for modules to import.

See section 1.2

Should that not be the same? And is it not better to set it: usr/lib/python/ How would I do that? Add it to the PYTHONPATH or set the PYTHONPATH to that? But how to set which python?

No they are not the same. You don't really need to modify the path to your Python interpreter. To modify the PYTHONPATH, you can set it in a shell, or from within a Python program by using sys.path

import sys
print(sys.path)
sys.path.append("another/path/to/search")