set pythonpath before import statements

This will add a path to your Python process / instance (i.e. the running executable). The path will not be modified for any other Python processes. Another running Python program will not have its path modified, and if you exit your program and run again the path will not include what you added before. What are you are doing is generally correct.

set.py:

import syssys.path.append("/tmp/TEST")

loop.py

import sysimport timewhile True:  print sys.path  time.sleep(1)

run: python loop.py &

This will run loop.py, connected to your STDOUT, and it will continue to run in the background. You can then run python set.py. Each has a different set of environment variables. Observe that the output from loop.py does not change because set.py does not change loop.py's environment.

A note on importing

Python imports are dynamic, like the rest of the language. There is no static linking going on. The import is an executable line, just like sys.path.append....

As also noted in the docs here.
Go to Python X.X/Lib and add these lines to the site.py there,

import syssys.path.append("yourpathstring")

This changes your sys.path so that on every load, it will have that value in it..

As stated here about site.py,

This module is automatically imported during initialization. Importing this module will append site-specific paths to the module search path and add a few builtins.

For other possible methods of adding some path to sys.path see these docs