code: def walk(self, top, topdown=True, onerror=None, followlinks=False):
          
          return walk(self.filesystem, top, topdown, onerror, followlinks)
docs: Perform a walk operation over the fake filesystem.
  
          Args:
              top: The root directory from which to begin walk.
              topdown: Determines whether to return the tuples with the root as
                  the first entry (`True`) or as the last, after all the child
                  directory tuples (`False`).
            onerror: If not `None`, function which will be called to handle the
                  `os.error` instance provided when `os.listdir()` fails.
            followlinks: If `True`, symbolic links are followed.
  
          Yields:
              (path, directories, nondirectories) for top and each of its
              subdirectories.  See the documentation for the builtin os module
              for further details.