code: def choose_backend(backend: Optional[str] = None) -> str:
      return configuration_of(
          ConfigVariable.QUERY_BACKEND, backend, "spark"
      )
  
imports: from typing import Optional, Union
  import sys
  import re
  from hailtop.config import ConfigVariable, configuration_of
label_desc: Use image node_id 1 for calling a global function with example usage: choose_backend(backend) and returns: configuration_of