Moves the reading cursor.

def Seek(self, offset, whence=os.SEEK_SET):
    """Moves the reading cursor."""

    if whence == os.SEEK_SET:
      self._offset = offset
    elif whence == os.SEEK_CUR:
      self._offset += offset
    elif whence == os.SEEK_END:
      self._offset = self._length + offset
    else:
      raise ValueError("Invalid whence argument: %s" % whence)