def test_common(self):

       other = self.root.join('sampledir')

->     x = other.common(self.root)

       assert x == self.root

   def common(self, other):

       """ return the common part shared with the other path

               or None if there is no common part.

           """

       last = None

       for x, y in zip(self.parts(), other.parts()):

->         if x != y:

               return last

           last = x

       return last