Allow numbers.Integral instead of only built-in int for QuantumRegister index

qiskit/circuit/register.py

@@ -19,6 +19,7 @@
 """
 import re
 import itertools
+import numbers
 
 from qiskit.circuit.exceptions import CircuitError
 
@@ -117,7 +118,7 @@ def __getitem__(self, key):
             CircuitError: if the `key` is not an integer.
             QiskitIndexError: if the `key` is not in the range `(0, self.size)`.
         """
-        if not isinstance(key, (int, slice, list)):
+        if not isinstance(key, (numbers.Integral, int, slice, list)):
             raise CircuitError("expected integer or slice index into register")
         if isinstance(key, slice):
             return self._bits[key]
@@ -152,4 +153,4 @@ def __eq__(self, other):
 
     def __hash__(self):
         """Make object hashable, based on the name and size to hash."""
-        return self._hash
+        return self._hash

releasenotes/notes/allowIntegerforQuantumRegisterSize-2e9a6cc033bc962e.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Use numbers.Integral instead of Python's built-in ``int`` as 
+    supported index to ``QuantumRegister`` and ``ClassicalRegister``.
+

test/python/circuit/test_circuit_registers.py

@@ -59,6 +59,18 @@ def test_qarg_numpy_int_size(self):
         self.assertEqual(qr1.size, 10)
         self.assertEqual(type(qr1), QuantumRegister)
 
+    def test_register_int_types(self):
+        """Test attempt to pass different types of integer as indices
+        of QuantumRegister and ClassicalRegister
+        """
+        ints = [int(2), np.int(2), np.int32(2), np.int64(2)]
+        for index in ints:
+            with self.subTest(index=index):
+                qr = QuantumRegister(4)
+                cr = ClassicalRegister(4)
+                self.assertEqual(qr[index], qr[2])
+                self.assertEqual(cr[index], cr[2])
+
     def test_numpy_array_of_registers(self):
         """Test numpy array of Registers .
         See https://github.com/Qiskit/qiskit-terra/issues/1898