code: def __getitem__(self, item):
      columns = self.input.dtypes.index
      if isinstance(item, (list, tuple)):
          item = list(item)
          for col in item:
              if col not in columns:
                  raise KeyError(f"Column not found: {col}")
      else:
          if item not in columns:
              raise KeyError(f"Column not found: {item}")
  
      return type(self)(input=self.input[item], **self.params)
  
file_path: mars/mars/dataframe/window/core.py
imports: from ...serialization.serializables import Serializable, KeyField
label_desc: Use image node_id 7 for calling the Window obj's underlying member method code with example usage: obj.__getitem__(item) and returns: type