code: def _iter_symbols(code):
          
          for node in ast.walk(ast.parse(code)):
              if isinstance(node, ast.Name):
                  yield node.id
docs: Iterate all the variable names in the given expression.
  
          Example:
  
          * ``self.foobar`` yields ``self``
          * ``self[foobar]`` yields `self`` and ``foobar``