How to use "/" (directory separator) in both Linux and Windows in Python?

Use os.path.join(). Example: os.path.join(pathfile,"output","log.txt").

In your code that would be: rootTree.write(os.path.join(pathfile,"output","log.txt"))

Use:

import os
print os.sep

to see how separator looks on a current OS.
In your code you can use:

import os
path = os.path.join('folder_name', 'file_name')

You can use os.sep:

>>> import os
>>> os.sep
'/'

os.path.normpath(pathname) should also be mentioned as it converts / path separators into \ separators on Windows. It also collapses redundant uplevel references... i.e., A/B and A/foo/../B and A/./B all become A/B. And if you are Windows, these all become A\B.