Add singleton lookup-key logic

This adds overridable logic for singleton classes to define a "key"
function, which takes the same arguments as its `__init__` and returns a
lookup key that is used as a comparison for returning singletons.  This
lets the standard singleton gates easily return the singleton instance
even when `label` is explicitly set to the default, such as happens
using the idiomatic `QuantumCircuit` methods like `x`.

This keying logic has a simple logical extension to allow more than one
singleton to exist for a single base class.  This functionality is not
yet used by the Qiskit standard library, but can easily be used to
make both closed- and open-controlled versions of gates singletons, or
to allow specific hard-coded parameters for (e.g.) `RZGate` singletons.

The decision on which gates to extend this to can be done in follow-up
commits later.

qiskit/circuit/library/standard_gates/dcx.py

@@ -12,7 +12,7 @@
 
 """Double-CNOT gate."""
 
-from qiskit.circuit.singleton import SingletonGate
+from qiskit.circuit.singleton import SingletonGate, stdlib_singleton_key
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit._utils import with_gate_array
 
@@ -52,6 +52,8 @@ def __init__(self, label=None, *, duration=None, unit="dt"):
         """Create new DCX gate."""
         super().__init__("dcx", 2, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         """
         gate dcx a, b { cx a, b; cx b, a; }

qiskit/circuit/library/standard_gates/ecr.py

@@ -16,7 +16,7 @@
 
 from qiskit.circuit._utils import with_gate_array
 from qiskit.circuit.quantumregister import QuantumRegister
-from qiskit.circuit.singleton import SingletonGate
+from qiskit.circuit.singleton import SingletonGate, stdlib_singleton_key
 from .rzx import RZXGate
 from .x import XGate
 
@@ -88,6 +88,8 @@ def __init__(self, label=None, *, duration=None, unit="dt"):
         """Create new ECR gate."""
         super().__init__("ecr", 2, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         """
         gate ecr a, b { rzx(pi/4) a, b; x a; rzx(-pi/4) a, b;}

qiskit/circuit/library/standard_gates/h.py

@@ -14,7 +14,7 @@
 from math import sqrt, pi
 from typing import Optional, Union
 import numpy
-from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate
+from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate, stdlib_singleton_key
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit._utils import with_gate_array, with_controlled_gate_array
 
@@ -55,6 +55,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create new H gate."""
         super().__init__("h", 1, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         """
         gate h a { u2(0,pi) a; }
@@ -181,6 +183,8 @@ def __init__(
             _base_label=_base_label,
         )
 
+    _singleton_lookup_key = stdlib_singleton_key(num_ctrl_qubits=1)
+
     def _define(self):
         """
         gate ch a,b {

qiskit/circuit/library/standard_gates/i.py

@@ -13,7 +13,7 @@
 """Identity gate."""
 
 from typing import Optional
-from qiskit.circuit.singleton import SingletonGate
+from qiskit.circuit.singleton import SingletonGate, stdlib_singleton_key
 from qiskit.circuit._utils import with_gate_array
 
 
@@ -49,6 +49,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create new Identity gate."""
         super().__init__("id", 1, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def inverse(self):
         """Invert this gate."""
         return IGate()  # self-inverse

qiskit/circuit/library/standard_gates/iswap.py

@@ -16,7 +16,7 @@
 
 import numpy as np
 
-from qiskit.circuit.singleton import SingletonGate
+from qiskit.circuit.singleton import SingletonGate, stdlib_singleton_key
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit._utils import with_gate_array
 
@@ -89,6 +89,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create new iSwap gate."""
         super().__init__("iswap", 2, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         """
         gate iswap a,b {

qiskit/circuit/library/standard_gates/s.py

@@ -17,7 +17,7 @@
 
 import numpy
 
-from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate
+from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate, stdlib_singleton_key
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit._utils import with_gate_array, with_controlled_gate_array
 
@@ -61,6 +61,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create new S gate."""
         super().__init__("s", 1, [], label=label, duration=None, unit="dt")
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         """
         gate s a { u1(pi/2) a; }
@@ -124,6 +126,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create new Sdg gate."""
         super().__init__("sdg", 1, [], label=label, duration=None, unit="dt")
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         """
         gate sdg a { u1(-pi/2) a; }
@@ -205,6 +209,8 @@ def __init__(
             unit=unit,
         )
 
+    _singleton_lookup_key = stdlib_singleton_key(num_ctrl_qubits=1)
+
     def _define(self):
         """
         gate cs a,b { h b; cp(pi/2) a,b; h b; }
@@ -276,6 +282,8 @@ def __init__(
             unit=unit,
         )
 
+    _singleton_lookup_key = stdlib_singleton_key(num_ctrl_qubits=1)
+
     def _define(self):
         """
         gate csdg a,b { h b; cp(-pi/2) a,b; h b; }

qiskit/circuit/library/standard_gates/swap.py

@@ -14,7 +14,7 @@
 
 from typing import Optional, Union
 import numpy
-from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate
+from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate, stdlib_singleton_key
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit._utils import with_gate_array, with_controlled_gate_array
 
@@ -62,6 +62,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create new SWAP gate."""
         super().__init__("swap", 2, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         """
         gate swap a,b { cx a,b; cx b,a; cx a,b; }
@@ -213,6 +215,8 @@ def __init__(
             unit=unit,
         )
 
+    _singleton_lookup_key = stdlib_singleton_key(num_ctrl_qubits=1)
+
     def _define(self):
         """
         gate cswap a,b,c

qiskit/circuit/library/standard_gates/sx.py

@@ -14,7 +14,7 @@
 
 from math import pi
 from typing import Optional, Union
-from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate
+from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate, stdlib_singleton_key
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit._utils import with_gate_array, with_controlled_gate_array
 
@@ -66,6 +66,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create new SX gate."""
         super().__init__("sx", 1, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         """
         gate sx a { rz(-pi/2) a; h a; rz(-pi/2); }
@@ -145,6 +147,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create new SXdg gate."""
         super().__init__("sxdg", 1, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         """
         gate sxdg a { rz(pi/2) a; h a; rz(pi/2); }
@@ -245,6 +249,8 @@ def __init__(
             unit=unit,
         )
 
+    _singleton_lookup_key = stdlib_singleton_key(num_ctrl_qubits=1)
+
     def _define(self):
         """
         gate csx a,b { h b; cu1(pi/2) a,b; h b; }

qiskit/circuit/library/standard_gates/t.py

@@ -17,7 +17,7 @@
 
 import numpy
 
-from qiskit.circuit.singleton import SingletonGate
+from qiskit.circuit.singleton import SingletonGate, stdlib_singleton_key
 from qiskit.circuit.library.standard_gates.p import PhaseGate
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit._utils import with_gate_array
@@ -59,6 +59,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create new T gate."""
         super().__init__("t", 1, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         """
         gate t a { u1(pi/4) a; }
@@ -120,6 +122,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create new Tdg gate."""
         super().__init__("tdg", 1, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         """
         gate tdg a { u1(pi/4) a; }

qiskit/circuit/library/standard_gates/x.py

@@ -17,7 +17,7 @@
 import numpy
 from qiskit.utils.deprecation import deprecate_func
 from qiskit.circuit.controlledgate import ControlledGate
-from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate
+from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate, stdlib_singleton_key
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit._utils import _ctrl_state_to_int, with_gate_array, with_controlled_gate_array
 
@@ -75,6 +75,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create new X gate."""
         super().__init__("x", 1, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         """
         gate x a { u3(pi,0,pi) a; }
@@ -210,6 +212,8 @@ def __init__(
             unit=unit,
         )
 
+    _singleton_lookup_key = stdlib_singleton_key(num_ctrl_qubits=1)
+
     def control(
         self,
         num_ctrl_qubits: int = 1,
@@ -333,6 +337,8 @@ def __init__(
             unit=unit,
         )
 
+    _singleton_lookup_key = stdlib_singleton_key(num_ctrl_qubits=2)
+
     def _define(self):
         """
         gate ccx a,b,c
@@ -442,6 +448,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create a new simplified CCX gate."""
         super().__init__("rccx", 3, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         """
         gate rccx a,b,c
@@ -519,6 +527,8 @@ def __init__(
             unit=unit,
         )
 
+    _singleton_lookup_key = stdlib_singleton_key(num_ctrl_qubits=3)
+
     def _define(self):
         """
         gate c3sqrtx a,b,c,d
@@ -628,6 +638,8 @@ def __init__(
             unit=unit,
         )
 
+    _singleton_lookup_key = stdlib_singleton_key(num_ctrl_qubits=3)
+
     # seems like open controls not hapening?
     def _define(self):
         """
@@ -767,6 +779,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create a new RC3X gate."""
         super().__init__("rcccx", 4, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         """
         gate rc3x a,b,c,d
@@ -859,6 +873,8 @@ def __init__(
             unit=unit,
         )
 
+    _singleton_lookup_key = stdlib_singleton_key(num_ctrl_qubits=4)
+
     # seems like open controls not hapening?
     def _define(self):
         """

qiskit/circuit/library/standard_gates/y.py

@@ -16,7 +16,7 @@
 from typing import Optional, Union
 
 # pylint: disable=cyclic-import
-from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate
+from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate, stdlib_singleton_key
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit._utils import with_gate_array, with_controlled_gate_array
 
@@ -74,6 +74,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create new Y gate."""
         super().__init__("y", 1, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         # pylint: disable=cyclic-import
         from qiskit.circuit.quantumcircuit import QuantumCircuit
@@ -195,6 +197,8 @@ def __init__(
             unit=unit,
         )
 
+    _singleton_lookup_key = stdlib_singleton_key(num_ctrl_qubits=1)
+
     def _define(self):
         """
         gate cy a,b { sdg b; cx a,b; s b; }

qiskit/circuit/library/standard_gates/z.py

@@ -18,7 +18,7 @@
 import numpy
 
 from qiskit.circuit._utils import with_gate_array, with_controlled_gate_array
-from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate
+from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate, stdlib_singleton_key
 from qiskit.circuit.quantumregister import QuantumRegister
 
 from .p import PhaseGate
@@ -77,6 +77,8 @@ def __init__(self, label: Optional[str] = None, *, duration=None, unit="dt"):
         """Create new Z gate."""
         super().__init__("z", 1, [], label=label, duration=duration, unit=unit)
 
+    _singleton_lookup_key = stdlib_singleton_key()
+
     def _define(self):
         # pylint: disable=cyclic-import
         from qiskit.circuit.quantumcircuit import QuantumCircuit
@@ -180,6 +182,8 @@ def __init__(
             unit=unit,
         )
 
+    _singleton_lookup_key = stdlib_singleton_key(num_ctrl_qubits=1)
+
     def _define(self):
         """
         gate cz a,b { h b; cx a,b; h b; }
@@ -265,6 +269,8 @@ def __init__(
             unit=unit,
         )
 
+    _singleton_lookup_key = stdlib_singleton_key(num_ctrl_qubits=2)
+
     def _define(self):
         """
         gate ccz a,b,c { h c; ccx a,b,c; h c; }