code: def build_expression(self, attribute: str) -> Optional[Expression]:
          
  
          expression_string = self._spec.get(attribute, None)
          if expression_string:
              try:
                  return Expression(str(expression_string))
              except Exception as err:
                  self.add_errors(
                      InvalidExpressionError(self.fully_qualified_name, self._spec, attribute, err))
  
          return None
docs: Builds an expression object.  Adds an error if expression creation has errors.