Fix sympify() for expressions with vector element .subs() (#14641) (#14642)

This commit fixes an issue in the .sympify() method when building the
Sympy expression for a `ParameterExpression` that was constructed by
calling `ParameterExpression.subs()` with a ParameterVectorElement type.
The previous implementation of this method was passing the symbols for
substitution by string which required sympy to "parse" the input. The
square brackets for indexing the vector element in it's string
representation was not valid for sympy to parse and this caused the
error. To avoid this problem this commit adjusts the usage to pass a
sypy expression object instead of a string. This is the intent of the
function as string parsing has other issues in sympy and this was an
oversight in the implementation that was doing this. By passing a sympy
object to sympy it is able to handle the vector's name with square
brackets.

Fixes #14640

(cherry picked from commit 69d7dbd)

Co-authored-by: Matthew Treinish <[email protected]>

Author: mergify[bot]

qiskit/circuit/parameterexpression.py

@@ -683,8 +683,8 @@ def sympify(self):
                 sympy_binds = {}
                 for old, new in inst.binds.items():
                     if isinstance(new, ParameterExpression):
-                        new = new.name
-                    sympy_binds[old.name] = new
+                        new = new.sympify()
+                    sympy_binds[old.sympify()] = new
                 output = output.subs(sympy_binds, simultaneous=True)
                 continue
 

test/python/circuit/test_parameter_expression.py

@@ -505,3 +505,17 @@ def test_sympify_all_ops(self):
         expected = sympy.Abs(expected)
         expected = expected.subs({c: a})
         self.assertEqual(result, expected)
+
+    @unittest.skipUnless(HAS_SYMPY, "Sympy is required for this test")
+    def test_sympify_subs_vector(self):
+        """Test an expression with subbed ParameterVectorElements is sympifiable"""
+        import sympy
+
+        p_vec = ParameterVector("p", length=2)
+        theta = Parameter("theta")
+
+        expression = theta + 1
+        expression = expression.subs({theta: p_vec[0]})
+        result = expression.sympify()
+        expected = sympy.Symbol("p[0]") + 1
+        self.assertEqual(expected, result)