text: import unittest
  
  import pystan
  from pystan._compat import PY2
  
  
  class TestUnconstrainPars(unittest.TestCase):
  
      @classmethod
      def setUpClass(cls):
          cls.model = pystan.StanModel(model_code="parameters { real x; } model { }")
  
      def test_unconstrain_pars(self):
          data, seed = {}, 1
          fit = self.model.fit_class(data, seed)
          assertRaisesRegex = self.assertRaisesRegexp if PY2 else self.assertRaisesRegex
          with assertRaisesRegex(RuntimeError, 'Variable x missing'):
              fit.unconstrain_pars({})
  