code: def is_alpha(self):
          
          if not isinstance(self.val, str_types):
              raise TypeError('val is not a string')
          if len(self.val) == 0:
              raise ValueError('val is empty')
          if not self.val.isalpha():
              self._err('Expected <%s> to contain only alphabetic chars, but did not.' % self.val)
          return self
docs: Asserts that val is non-empty string and all characters are alphabetic.