Steve Dower added the comment:
I'm afraid not. The ._pth file is intended for embedding applications that have
a static set of search paths.
You may want to try adding a startup file (search for PYTHONSTARTUP) that
modifies sys.path directly. Or alternatively if you rename the ._pth file to
New submission from emve :
When I add mylib to python39._pth, Python treats
it relatively to c:\Python3 (where my embedded python resides).
>>> import sys
>>> print('\n'.join(sys.path))
c:\python3\python39.zip
c:\python3
c:\python3\lib
c:\python3\mylib
c:\python3\lib\site-packages
>>>
However,
