code: def test_regular_halflife_std():
      primitive_instance = ExponentialWeightedSTD(halflife=2.7)
      primitive_func = primitive_instance.get_function()
      array = pd.Series([1, 2, 7, 5])
      answer = pd.Series(primitive_func(array))
      correct_answer = pd.Series(
          [
              np.nan,
              0.7071067811865475,
              3.3565236098585416,
              2.631776826295855,
          ],
      )
      pd.testing.assert_series_equal(answer, correct_answer)
  
imports: import numpy
  import pandas
  from featuretools.primitives import ExponentialWeightedAverage, ExponentialWeightedSTD, ExponentialWeightedVariance
label_desc: Use image node_id 9 for calling a global function with example usage: test_regular_halflife_std() without return types