code: def is_length(self, length):
          
          if type(length) is not int:
              raise TypeError('given arg must be an int')
          if length < 0:
              raise ValueError('given arg must be a positive int')
          if len(self.val) != length:
              self._err('Expected <%s> to be of length <%d>, but was <%d>.' % (self.val, length, len(self.val)))
          return self
docs: Asserts that val is the given length.