class_docstr: Iterative lookup in a stack of dicts
  
  Assignments go into an internal dict that is also the first place
  where a lookup is done.
class_docstr_tok: ["Iterative","lookup","in","a","stack","of","dicts","Assignments","go","into","an","internal","dict","that","is","also","the","first","place","where","a","lookup","is","done","."]
docstr: Number of unique keys
docstr_tok: ["Number","of","unique","keys"]
code: def __len__(self):
      """
      Number of unique keys
      """
      return len({key for d in self.stack for key in d})
  
imports: from __future__ import annotations
  from collections.abc import MutableMapping
  from contextlib import suppress
  from dataclasses import dataclass
  from functools import lru_cache
  from typing import TYPE_CHECKING
label_desc: Use image node_id 7 for calling the StackedLookup obj's underlying member method code with example usage: obj.__len__() and returns: len