code: def test_window_sum(stream):
      df = cudf.DataFrame({"x": [1, 2, 3], "y": [4, 5, 6]})
      sdf = DataFrame(example=df, stream=stream)
      L = sdf.window(n=4).x.sum().stream.gather().sink_to_list()
  
      sdf.emit(df)
      assert L == [6]
      sdf.emit(df)
      assert L == [6, 9]
      sdf.emit(df)
      assert L == [6, 9, 9]
  
imports: import json
  import operator
  import numpy
  import pandas
  import pytest
  from dask.dataframe.utils import assert_eq
  from distributed import Client
  from streamz import Stream
  from streamz.dask import DaskStream
  from streamz.dataframe import Aggregation, DataFrame, DataFrames, Series
label_desc: Use image node_id 31 for calling a global function with example usage: test_window_sum(stream) without return types