Revert "Support qubits in MeasurementKey and update JSON serializat…

…ion of keys" (quantumlib#4277)

* Revert "Support qubits in `MeasurementKey` and update JSON serialization of keys (quantumlib#4123)"

This reverts commit 81436fe.

* Safeguard against repr-inequality.

* assert_equivalent_repr

cirq-core/cirq/__init__.py

@@ -433,7 +433,6 @@
     canonicalize_half_turns,
     chosen_angle_to_canonical_half_turns,
     chosen_angle_to_half_turns,
-    default_measurement_key_str,
     Duration,
     DURATION_LIKE,
     GenericMetaImplementAnyOneOf,

cirq-core/cirq/circuits/circuit_test.py

@@ -1367,16 +1367,8 @@ def test_findall_operations_with_gate(circuit_cls):
         (3, cirq.CZ(a, b), cirq.CZ),
     ]
     assert list(c.findall_operations_with_gate_type(cirq.MeasurementGate)) == [
-        (
-            4,
-            cirq.MeasurementGate(1).on(a),
-            cirq.MeasurementGate(1, key=cirq.MeasurementKey(qubits=(a,))),
-        ),
-        (
-            4,
-            cirq.MeasurementGate(1).on(b),
-            cirq.MeasurementGate(1, key=cirq.MeasurementKey(qubits=(b,))),
-        ),
+        (4, cirq.MeasurementGate(1, key='a').on(a), cirq.MeasurementGate(1, key='a')),
+        (4, cirq.MeasurementGate(1, key='b').on(b), cirq.MeasurementGate(1, key='b')),
     ]
 
 

cirq-core/cirq/ops/gate_operation.py

@@ -123,10 +123,6 @@ def __str__(self) -> str:
     def _json_dict_(self) -> Dict[str, Any]:
         return protocols.obj_to_dict_helper(self, ['gate', 'qubits'])
 
-    @classmethod
-    def _from_json_dict_(cls, gate, qubits, **kwargs):
-        return gate.on(*qubits)
-
     def _group_interchangeable_qubits(
         self,
     ) -> Tuple[Union['cirq.Qid', Tuple[int, FrozenSet['cirq.Qid']]], ...]:

cirq-core/cirq/ops/measure_util.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Callable, List, Tuple, TYPE_CHECKING, Union
+from typing import Callable, Iterable, List, Optional, Tuple, TYPE_CHECKING, Union
 
 import numpy as np
 
@@ -24,9 +24,13 @@
     import cirq
 
 
+def _default_measurement_key(qubits: Iterable[raw_types.Qid]) -> str:
+    return ','.join(str(q) for q in qubits)
+
+
 def measure(
     *target: 'cirq.Qid',
-    key: Union[str, value.MeasurementKey] = '',
+    key: Optional[Union[str, value.MeasurementKey]] = None,
     invert_mask: Tuple[bool, ...] = (),
 ) -> raw_types.Operation:
     """Returns a single MeasurementGate applied to all the given qubits.
@@ -35,7 +39,7 @@ def measure(
 
     Args:
         *target: The qubits that the measurement gate should measure.
-        key: The string key of the measurement. If this is empty, defaults
+        key: The string key of the measurement. If this is None, it defaults
             to a comma-separated list of the target qubits' str values.
         invert_mask: A list of Truthy or Falsey values indicating whether
             the corresponding qubits should be flipped. None indicates no
@@ -56,12 +60,14 @@ def measure(
         elif not isinstance(qubit, raw_types.Qid):
             raise ValueError('measure() was called with type different than Qid.')
 
+    if key is None:
+        key = _default_measurement_key(target)
     qid_shape = protocols.qid_shape(target)
     return MeasurementGate(len(target), key, invert_mask, qid_shape).on(*target)
 
 
 def measure_each(
-    *qubits: 'cirq.Qid', key_func: Callable[[raw_types.Qid], str] = lambda x: ''
+    *qubits: 'cirq.Qid', key_func: Callable[[raw_types.Qid], str] = str
 ) -> List[raw_types.Operation]:
     """Returns a list of operations individually measuring the given qubits.
 
@@ -70,9 +76,7 @@ def measure_each(
     Args:
         *qubits: The qubits to measure.
         key_func: Determines the key of the measurements of each qubit. Takes
-            the qubit and returns the key for that qubit. Defaults to empty string
-            key which would result in a comma-separated list of the target qubits'
-            str values.
+            the qubit and returns the key for that qubit. Defaults to str.
 
     Returns:
         A list of operations individually measuring the given qubits.

cirq-core/cirq/ops/measure_util_test.py

@@ -26,22 +26,12 @@ def test_measure_qubits():
     with pytest.raises(ValueError, match='empty set of qubits'):
         _ = cirq.measure()
 
-    assert cirq.measure(a) == cirq.MeasurementGate(num_qubits=1, key='').on(a)
-    assert cirq.measure(a) != cirq.measure(a, key='a')
-    assert cirq.measure(a) != cirq.MeasurementGate(num_qubits=1, key='').on(b)
-    assert cirq.measure(a) != cirq.MeasurementGate(num_qubits=1, key='a').on(a)
-    assert cirq.measure(a) == cirq.MeasurementGate(
-        num_qubits=1, key=cirq.MeasurementKey(qubits=(a,))
-    ).on(a)
-    assert cirq.measurement_key(cirq.measure(a)) == 'a'
-    assert cirq.measure(a, b) == cirq.MeasurementGate(num_qubits=2, key='').on(a, b)
-    assert cirq.measurement_key(cirq.measure(a, b)) == 'a,b'
-    assert cirq.measure(b, a) == cirq.MeasurementGate(num_qubits=2, key='').on(b, a)
-    assert cirq.measurement_key(cirq.measure(b, a)) == 'b,a'
+    assert cirq.measure(a) == cirq.MeasurementGate(num_qubits=1, key='a').on(a)
+    assert cirq.measure(a, b) == cirq.MeasurementGate(num_qubits=2, key='a,b').on(a, b)
+    assert cirq.measure(b, a) == cirq.MeasurementGate(num_qubits=2, key='b,a').on(b, a)
     assert cirq.measure(a, key='b') == cirq.MeasurementGate(num_qubits=1, key='b').on(a)
-    assert cirq.measure(a, key='b') != cirq.MeasurementGate(num_qubits=1, key='b').on(b)
     assert cirq.measure(a, invert_mask=(True,)) == cirq.MeasurementGate(
-        num_qubits=1, invert_mask=(True,)
+        num_qubits=1, key='a', invert_mask=(True,)
     ).on(a)
     assert cirq.measure(*cirq.LineQid.for_qid_shape((1, 2, 3)), key='a') == cirq.MeasurementGate(
         num_qubits=3, key='a', qid_shape=(1, 2, 3)

cirq-core/cirq/ops/measurement_gate.py

@@ -12,12 +12,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Any, Dict, Optional, Tuple, Sequence, TYPE_CHECKING, Union
+from typing import Any, Dict, Iterable, Optional, Tuple, Sequence, TYPE_CHECKING, Union
 
 import numpy as np
 
 from cirq import protocols, value
-from cirq.ops import raw_types, gate_operation
+from cirq.ops import raw_types
 
 if TYPE_CHECKING:
     import cirq
@@ -80,21 +80,12 @@ def key(self, key: Union[str, value.MeasurementKey]):
         else:
             self.mkey = value.MeasurementKey(name=key)
 
-    def on(self, *qubits: raw_types.Qid) -> raw_types.Operation:
-        """Returns an application of this gate to the given qubits.
-
-        Args:
-            *qubits: The collection of qubits to potentially apply the gate to.
-        """
-        maybe_rekeyed_gate = self.with_key(self.mkey.with_qubits(qubits))
-        return gate_operation.GateOperation(maybe_rekeyed_gate, list(qubits))
-
     def _qid_shape_(self) -> Tuple[int, ...]:
         return self._qid_shape
 
     def with_key(self, key: Union[str, value.MeasurementKey]) -> 'MeasurementGate':
         """Creates a measurement gate with a new key but otherwise identical."""
-        if isinstance(key, value.MeasurementKey) and key == self.mkey:
+        if key == self.key:
             return self
         return MeasurementGate(
             self.num_qubits(), key=key, invert_mask=self.invert_mask, qid_shape=self._qid_shape
@@ -114,7 +105,7 @@ def with_bits_flipped(self, *bit_positions: int) -> 'MeasurementGate':
         for b in bit_positions:
             new_mask[b] = not new_mask[b]
         return MeasurementGate(
-            self.num_qubits(), key=self.mkey, invert_mask=tuple(new_mask), qid_shape=self._qid_shape
+            self.num_qubits(), key=self.key, invert_mask=tuple(new_mask), qid_shape=self._qid_shape
         )
 
     def full_invert_mask(self):
@@ -161,8 +152,8 @@ def _circuit_diagram_info_(
                 if b:
                     symbols[i] = '!M'
 
-        # Mention the measurement key if it is non-trivial or there are no known qubits.
-        if self.mkey.name or self.mkey.path or not args.known_qubits:
+        # Mention the measurement key.
+        if not args.known_qubits or self.key != _default_measurement_key(args.known_qubits):
             symbols[0] += f"('{self.key}')"
 
         return protocols.CircuitDiagramInfo(tuple(symbols))
@@ -200,13 +191,8 @@ def _quil_(
 
     def _op_repr_(self, qubits: Sequence['cirq.Qid']) -> str:
         args = list(repr(q) for q in qubits)
-        if self.mkey.name or self.mkey.path:
-            if self.mkey == self.mkey.name:
-                args.append(f'key={self.mkey.name!r}')
-            else:
-                # Remove qubits from the `MeasurementKey` representation since we already have
-                # qubits from the op.
-                args.append(f'key={self.mkey.with_qubits(tuple())!r}')
+        if self.key != _default_measurement_key(qubits):
+            args.append(f'key={self.key!r}')
         if self.invert_mask:
             args.append(f'invert_mask={self.invert_mask!r}')
         arg_list = ', '.join(args)
@@ -219,13 +205,13 @@ def __repr__(self):
         return (
             f'cirq.MeasurementGate('
             f'{self.num_qubits()!r}, '
-            f'{self.mkey.name if self.mkey == self.mkey.name else self.mkey!r}, '
+            f'{self.key!r}, '
             f'{self.invert_mask}'
             f'{qid_shape_arg})'
         )
 
     def _value_equality_values_(self) -> Any:
-        return self.mkey, self.invert_mask, self._qid_shape
+        return self.key, self.invert_mask, self._qid_shape
 
     def _json_dict_(self) -> Dict[str, Any]:
         other = {}
@@ -234,7 +220,7 @@ def _json_dict_(self) -> Dict[str, Any]:
         return {
             'cirq_type': self.__class__.__name__,
             'num_qubits': len(self._qid_shape),
-            'key': self.mkey,
+            'key': self.key,
             'invert_mask': self.invert_mask,
             **other,
         }
@@ -243,7 +229,7 @@ def _json_dict_(self) -> Dict[str, Any]:
     def _from_json_dict_(cls, num_qubits, key, invert_mask, qid_shape=None, **kwargs):
         return cls(
             num_qubits=num_qubits,
-            key=value.MeasurementKey.parse_serialized(key) if isinstance(key, str) else key,
+            key=value.MeasurementKey.parse_serialized(key),
             invert_mask=tuple(invert_mask),
             qid_shape=None if qid_shape is None else tuple(qid_shape),
         )
@@ -254,3 +240,7 @@ def _has_stabilizer_effect_(self) -> Optional[bool]:
     def _act_on_(self, args: 'cirq.ActOnArgs', qubits: Sequence['cirq.Qid']) -> bool:
         args.measure(qubits, self.key, self.full_invert_mask())
         return True
+
+
+def _default_measurement_key(qubits: Iterable[raw_types.Qid]) -> str:
+    return ','.join(str(q) for q in qubits)

cirq-core/cirq/ops/measurement_gate_test.py

@@ -18,6 +18,15 @@
 import cirq
 
 
+def test_eval_repr():
+    # Basic safeguard against repr-inequality.
+    op = cirq.GateOperation(
+        gate=cirq.MeasurementGate(1, cirq.MeasurementKey(path=(), name='q0_1_0'), ()),
+        qubits=[cirq.GridQubit(0, 1)],
+    )
+    cirq.testing.assert_equivalent_repr(op)
+
+
 @pytest.mark.parametrize('num_qubits', [1, 2, 4])
 def test_measure_init(num_qubits):
     assert cirq.MeasurementGate(num_qubits).num_qubits() == num_qubits
@@ -266,15 +275,6 @@ def test_op_repr():
         "key='out', "
         "invert_mask=(False, True))"
     )
-    assert repr(
-        cirq.measure(
-            a, b, key=cirq.MeasurementKey(name='out', path=('a', 'b')), invert_mask=(False, True)
-        )
-    ) == (
-        "cirq.measure(cirq.LineQubit(0), cirq.LineQubit(1), "
-        "key=cirq.MeasurementKey(path=('a', 'b'), name='out'), "
-        "invert_mask=(False, True))"
-    )
 
 
 def test_act_on_state_vector():

cirq-core/cirq/protocols/json_test_data/Circuit.json

@@ -85,33 +85,7 @@
             "gate": {
               "cirq_type": "MeasurementGate",
               "num_qubits": 5,
-              "key":  {
-                "cirq_type": "MeasurementKey",
-                "name": "",
-                "path": [],
-                "qubits": [
-                  {
-                    "cirq_type": "LineQubit",
-                    "x": 0
-                  },
-                  {
-                    "cirq_type": "LineQubit",
-                    "x": 1
-                  },
-                  {
-                    "cirq_type": "LineQubit",
-                    "x": 2
-                  },
-                  {
-                    "cirq_type": "LineQubit",
-                    "x": 3
-                  },
-                  {
-                    "cirq_type": "LineQubit",
-                    "x": 4
-                  }
-                ]
-              },
+              "key": "0,1,2,3,4",
               "invert_mask": []
             },
             "qubits": [