docs: def get_value(self, node):
          """Convert value from an AST node."""
          if not isinstance(node, ast.Attribute):
              raise TypeError("must be an attribute")
  
          if node.value.id != self.choices.__name__:
              raise TypeError("must be an attribute of {}".format(self.choices.__name__))
  
          return getattr(self.choices, node.attr)