class_docstr: A metaclass that creates a Singleton base class when called. 
class_docstr_tok: ["A","metaclass","that","creates","a","Singleton","base","class","when","called","."]
code: def __call__(cls, *args):
      if cls not in cls._instances:
          cls._instances[cls] = super(_Singleton, cls).__call__(*args)
      return cls._instances[cls]
  
imports: import json
  from enum import Enum
  from typing import Union
  from pyspark.sql import Column
  import pyspark.sql.functions
label_desc: Use image node_id 1 for calling the _Singleton obj's underlying member method code with example usage: obj.__call__(cls) and returns: cls