code: def test_ar_to_impulse(self):
      arp = np.array([0.9])
      impulses = _ar_to_impulse(20, arp)
      expected = 0.9 ** np.arange(20)
      assert_allclose(impulses, expected)
  
      arp = np.array([0.5, 0.3])
      impulses = _ar_to_impulse(20, arp)
      comp = np.array([arp, [1, 0]])
      a = comp.copy()
      expected = np.ones(20)
      for i in range(1, 20):
          expected[i] = a[0, 0]
          a = a.dot(comp)
      assert_allclose(impulses, expected)
  
      arp = np.array([1.5, 0.0, -0.7])
      impulses = _ar_to_impulse(20, arp)
      comp = np.array([arp, [1, 0, 0], [0, 1, 0]])
      a = comp.copy()
      expected = np.ones(20)
      for i in range(1, 20):
          expected[i] = a[0, 0]
          a = a.dot(comp)
      assert_allclose(impulses, expected)
  
imports: import datetime
  from itertools import product
  import numpy
  from numpy.random import RandomState
  from numpy.testing import assert_allclose
  import pandas
  from pandas.testing import assert_frame_equal
  import pytest
  from arch.data import sp500
  from arch.tests.univariate.test_variance_forecasting import preserved_state
  from arch.univariate import APARCH, ARX, EGARCH, FIGARCH, GARCH, HARCH, HARX, ConstantMean, ConstantVariance, EWMAVariance, MIDASHyperbolic, RiskMetrics2006, ZeroMean, arch_model
  from arch.univariate.mean import _ar_forecast, _ar_to_impulse
label_desc: Use image node_id 3 for calling the TestForecasting obj's underlying member method code with example usage: obj.test_ar_to_impulse() without return types