Today I found great tutorial that explain how sys.path is working. Follow this link to see through :
If a script is executed, the interpreter sets the first entry of sys.path to that script’s directory. If Python is launched interactively, the first entry is the empty string (“”), meaning Python will scan the present working directory first. The next entries of sys.path are the contents of the PYTHONPATH environment variable, if it exists. Then, installation-dependent entries are appended (example below).
When initializing, the interpreter normally imports the site module automatically. The module, on import, executes code to find .pth files in known site-packages directory locations, which themselves contain entries which are either paths to add to sys.path, or import calls. If we really want to trace what’s going on, we can launch a Python interpreter with -S to prevent loading the site module automatically, and instead trace the import.