So I have this python code:

print os.path.commonprefix([r'C:\root\dir',r'C:\root\dir1'])

Real Result

C:\root\dir

Desired result

C:\root

Question 1

Based on os.path.commonprefix documentation:
Return the longest path prefix (taken character-by-character)

Is there a similar function that:
Return the longest path prefix (taken dir by dir)

Question 2

if commonprefix is implemented in os.path why isn't it path oriented, meaning return my desired result and not the real one?

Note:

I can implement this easily by myself but if it is already implemented why not using it?

is there a path oriented function?

no and yes. commonprefix() can work with arbitrary sequences, not just strings.

Split the path into components and call commonprefix() on that e.g.:

>>> import os
>>> from pathlib import PureWindowsPath
>>> a, b = map(PureWindowsPath, [r'C:\root\dir', r'C:\root\dir1'])
>>> PureWindowsPath(*os.path.commonprefix([a.parts, b.parts]))
PureWindowsPath('C:/root')