code: def is_empty(self):
          
          if len(self.val) != 0:
              if isinstance(self.val, str_types):
                  self._err('Expected <%s> to be empty string, but was not.' % self.val)
              else:
                  self._err('Expected <%s> to be empty, but was not.' % self.val)
          return self
docs: Asserts that val is empty.