class_docstr: Class for storing a causal estimand, typically as a result of the identification step.
code: def set_backdoor_variables(
      self, bdoor_variables_arr: List, key: Optional[str] = None
  ):
      if key is None:
          key = self.identifier_method
      self.backdoor_variables[key] = bdoor_variables_arr
  
file_path: dowhy/dowhy/causal_identifier/identified_estimand.py
imports: import copy
  from typing import List, Optional
  import sympy
  from dowhy.utils.api import parse_state
label_desc: Use image node_id 4 for calling the IdentifiedEstimand obj's underlying member method code with example usage: obj.set_backdoor_variables(bdoor_variables_arr, key) without return types