code: def _check_engine(engine):
      
      from pandas.core.computation.check import _NUMEXPR_INSTALLED
  
      if engine is None:
          if _NUMEXPR_INSTALLED:
              engine = 'numexpr'
          else:
              engine = 'python'
  
      if engine not in _engines:
          valid = list(_engines.keys())
          raise KeyError('Invalid engine {engine!r} passed, valid engines are'
                         ' {valid}'.format(engine=engine, valid=valid))
  
      # TODO: validate this in a more general way (thinking of future engines
      # that won't necessarily be import-able)
      # Could potentially be done on engine instantiation
      if engine == 'numexpr':
          if not _NUMEXPR_INSTALLED:
              raise ImportError("'numexpr' is not installed or an "
                                "unsupported version. Cannot use "
                                "engine='numexpr' for query/eval "
                                "if 'numexpr' is not installed")
  
      return engine
docs: Make sure a valid engine is passed.
  
      Parameters
      ----------
      engine : str
  
      Raises
      ------
      KeyError
        * If an invalid engine is passed
      ImportError
        * If numexpr was requested but doesn't exist
  
      Returns
      -------
      string engine