code: def test_arx_no_lags():
      mod = ARX(SP500, volatility=GARCH())
      res = mod.fit(disp="off")
      assert res.params.shape[0] == 4
      assert "lags" not in mod._model_description(include_lags=False)
  
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 5 for calling a global function with example usage: test_arx_no_lags() without return types