code: def test_ewm_std():
      array = [1, np.nan, 3, 4, 5, 6, np.nan, 7]
  
      s = Stream.source(array, dtype="float")
  
      for config in configurations:
          w = s.ewm(**config).std(bias=False).rename("w")
          expected = list(
              pd.Series(array).ewm(**config).std(bias=False)
          )
  
          assert_op([w], expected)
  
      for config in configurations:
          w = s.ewm(**config).std(bias=True).rename("w")
          expected = list(pd.Series(array).ewm(**config).std(bias=True))
  
          assert_op([w], expected)
  
imports: import numpy
  import pandas
  from tensortrade.feed import Stream
  from tests.utils.ops import assert_op
label_desc: Use image node_id 3 for calling a global function with example usage: test_ewm_std() without return types