MPolynomial_element._test_subs: Check that partial substitution by elements from another ring raises an exception

Author: Matthias Koeppe

src/sage/rings/polynomial/multi_polynomial_element.py

@@ -402,14 +402,21 @@ def _test_subs(self, tester=None, **options):
             # with elements of another ring
             from sage.rings.polynomial.polynomial_ring_constructor import PolynomialRing
             other = PolynomialRing(self.parent().base_ring(), 'other', len(gens))
-            d = {str(gen): other_gen for gen, other_gen in zip(gens, other.gens())}
+            other_gens = other.gens()
+            d = {str(gen): other_gen for gen, other_gen in zip(gens, other_gens)}
             tester.assertEqual(self.subs(**d).parent(), other)
 
         if len(gens) > 1:
             # substituting one variable (in a polynomial ring with variables) with 0
             d = {str(gens[0]): 0}
             tester.assertEqual(self.subs(**d).parent(), self.parent())
 
+            # test error checking: partial substitution by elements
+            # from another ring is not allowed
+            d = {str(gens[0]): other_gens[0]}
+            with tester.assertRaises((ValueError, TypeError)):
+                self.subs(**d)
+
 
 class MPolynomial_polydict(Polynomial_singular_repr, MPolynomial_element):
     r"""