Add {Frozen}Circuit.from_moments to construct circuit by moments. (quantumlib#5805)

In our internal code almost all circuits are constructed with explicit moments rather than using an insertion strategy because we want to control the alignment of ops. Here we add a `from_moments` classmethod on `Circuit` and `FrozenCircuit` which takes any number of `OP_TREE` args and converts each one to a `Moment` in the resulting circuit. This gives a convenient way to construct circuits without cluttering the code with multiple calls to `Moment`.

Also adds special cases in both the `Circuit` and `FrozenCircuit` constructors to handle constructing from a single circuit or a sequence of moments.

Author: maffoo

cirq-core/cirq/circuits/circuit.py

@@ -143,6 +143,30 @@ class AbstractCircuit(abc.ABC):
     *   get_independent_qubit_sets
     """
 
+    @classmethod
+    def from_moments(cls: Type[CIRCUIT_TYPE], *moments: 'cirq.OP_TREE') -> CIRCUIT_TYPE:
+        """Create a circuit from moment op trees.
+
+        Args:
+            *moments: Op tree for each moment.
+        """
+        return cls._from_moments(
+            moment if isinstance(moment, Moment) else Moment(moment) for moment in moments
+        )
+
+    @classmethod
+    @abc.abstractmethod
+    def _from_moments(cls: Type[CIRCUIT_TYPE], moments: Iterable['cirq.Moment']) -> CIRCUIT_TYPE:
+        """Create a circuit from moments.
+
+        This must be implemented by subclasses. It provides a more efficient way
+        to construct a circuit instance since we already have the moments and so
+        can skip the analysis required to implement various insert strategies.
+
+        Args:
+            moments: Moments of the circuit.
+        """
+
     @property
     @abc.abstractmethod
     def moments(self) -> Sequence['cirq.Moment']:
@@ -225,8 +249,7 @@ def __getitem__(self: CIRCUIT_TYPE, key: Tuple[slice, Iterable['cirq.Qid']]) ->
 
     def __getitem__(self, key):
         if isinstance(key, slice):
-            sliced_moments = self.moments[key]
-            return self._with_sliced_moments(sliced_moments)
+            return self._from_moments(self.moments[key])
         if hasattr(key, '__index__'):
             return self.moments[key]
         if isinstance(key, tuple):
@@ -239,17 +262,12 @@ def __getitem__(self, key):
                 return selected_moments[qubit_idx]
             if isinstance(qubit_idx, ops.Qid):
                 qubit_idx = [qubit_idx]
-            sliced_moments = [moment[qubit_idx] for moment in selected_moments]
-            return self._with_sliced_moments(sliced_moments)
+            return self._from_moments(moment[qubit_idx] for moment in selected_moments)
 
         raise TypeError('__getitem__ called with key not of type slice, int, or tuple.')
 
     # pylint: enable=function-redefined
 
-    @abc.abstractmethod
-    def _with_sliced_moments(self: CIRCUIT_TYPE, moments: Iterable['cirq.Moment']) -> CIRCUIT_TYPE:
-        """Helper method for constructing circuits from __getitem__."""
-
     def __str__(self) -> str:
         return self.to_text_diagram()
 
@@ -909,7 +927,7 @@ def map_moment(moment: 'cirq.Moment') -> 'cirq.Circuit':
             """Apply func to expand each op into a circuit, then zip up the circuits."""
             return Circuit.zip(*[Circuit(func(op)) for op in moment])
 
-        return self._with_sliced_moments(m for moment in self for m in map_moment(moment))
+        return self._from_moments(m for moment in self for m in map_moment(moment))
 
     def qid_shape(
         self, qubit_order: 'cirq.QubitOrderOrList' = ops.QubitOrder.DEFAULT
@@ -949,18 +967,16 @@ def _measurement_key_names_(self) -> FrozenSet[str]:
         return self.all_measurement_key_names()
 
     def _with_measurement_key_mapping_(self, key_map: Mapping[str, str]):
-        return self._with_sliced_moments(
-            [protocols.with_measurement_key_mapping(moment, key_map) for moment in self.moments]
+        return self._from_moments(
+            protocols.with_measurement_key_mapping(moment, key_map) for moment in self.moments
         )
 
     def _with_key_path_(self, path: Tuple[str, ...]):
-        return self._with_sliced_moments(
-            [protocols.with_key_path(moment, path) for moment in self.moments]
-        )
+        return self._from_moments(protocols.with_key_path(moment, path) for moment in self.moments)
 
     def _with_key_path_prefix_(self, prefix: Tuple[str, ...]):
-        return self._with_sliced_moments(
-            [protocols.with_key_path_prefix(moment, prefix) for moment in self.moments]
+        return self._from_moments(
+            protocols.with_key_path_prefix(moment, prefix) for moment in self.moments
         )
 
     def _with_rescoped_keys_(
@@ -971,7 +987,7 @@ def _with_rescoped_keys_(
             new_moment = protocols.with_rescoped_keys(moment, path, bindable_keys)
             moments.append(new_moment)
             bindable_keys |= protocols.measurement_key_objs(new_moment)
-        return self._with_sliced_moments(moments)
+        return self._from_moments(moments)
 
     def _qid_shape_(self) -> Tuple[int, ...]:
         return self.qid_shape()
@@ -1552,9 +1568,7 @@ def factorize(self: CIRCUIT_TYPE) -> Iterable[CIRCUIT_TYPE]:
         # the qubits from one factor belong to a specific independent qubit set.
         # This makes it possible to create independent circuits based on these
         # moments.
-        return (
-            self._with_sliced_moments([m[qubits] for m in self.moments]) for qubits in qubit_factors
-        )
+        return (self._from_moments(m[qubits] for m in self.moments) for qubits in qubit_factors)
 
     def _control_keys_(self) -> FrozenSet['cirq.MeasurementKey']:
         controls = frozenset(k for op in self.all_operations() for k in protocols.control_keys(op))
@@ -1719,6 +1733,12 @@ def __init__(
             else:
                 self.append(contents, strategy=strategy)
 
+    @classmethod
+    def _from_moments(cls, moments: Iterable['cirq.Moment']) -> 'Circuit':
+        new_circuit = Circuit()
+        new_circuit._moments[:] = moments
+        return new_circuit
+
     def _load_contents_with_earliest_strategy(self, contents: 'cirq.OP_TREE'):
         """Optimized algorithm to load contents quickly.
 
@@ -1813,11 +1833,6 @@ def copy(self) -> 'Circuit':
         copied_circuit._moments = self._moments[:]
         return copied_circuit
 
-    def _with_sliced_moments(self, moments: Iterable['cirq.Moment']) -> 'Circuit':
-        new_circuit = Circuit()
-        new_circuit._moments = list(moments)
-        return new_circuit
-
     # pylint: disable=function-redefined
     @overload
     def __setitem__(self, key: int, value: 'cirq.Moment'):

cirq-core/cirq/circuits/circuit_test.py

@@ -70,6 +70,23 @@ def validate_moment(self, moment):
 moment_and_op_type_validating_device = _MomentAndOpTypeValidatingDeviceType()
 
 
+def test_from_moments():
+    a, b, c, d = cirq.LineQubit.range(4)
+    assert cirq.Circuit.from_moments(
+        [cirq.X(a), cirq.Y(b)],
+        [cirq.X(c)],
+        [],
+        cirq.Z(d),
+        [cirq.measure(a, b, key='ab'), cirq.measure(c, d, key='cd')],
+    ) == cirq.Circuit(
+        cirq.Moment(cirq.X(a), cirq.Y(b)),
+        cirq.Moment(cirq.X(c)),
+        cirq.Moment(),
+        cirq.Moment(cirq.Z(d)),
+        cirq.Moment(cirq.measure(a, b, key='ab'), cirq.measure(c, d, key='cd')),
+    )
+
+
 def test_alignment():
     assert repr(cirq.Alignment.LEFT) == 'cirq.Alignment.LEFT'
     assert repr(cirq.Alignment.RIGHT) == 'cirq.Alignment.RIGHT'
@@ -269,6 +286,16 @@ def test_append_control_key_subcircuit():
     assert len(c) == 1
 
 
+def test_measurement_key_paths():
+    a = cirq.LineQubit(0)
+    circuit1 = cirq.Circuit(cirq.measure(a, key='A'))
+    assert cirq.measurement_key_names(circuit1) == {'A'}
+    circuit2 = cirq.with_key_path(circuit1, ('B',))
+    assert cirq.measurement_key_names(circuit2) == {'B:A'}
+    circuit3 = cirq.with_key_path_prefix(circuit2, ('C',))
+    assert cirq.measurement_key_names(circuit3) == {'C:B:A'}
+
+
 def test_append_moments():
     a = cirq.NamedQubit('a')
     b = cirq.NamedQubit('b')

cirq-core/cirq/circuits/frozen_circuit.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 """An immutable version of the Circuit data structure."""
-from typing import TYPE_CHECKING, FrozenSet, Iterable, Iterator, Sequence, Tuple, Union
+from typing import FrozenSet, Iterable, Iterator, Sequence, Tuple, TYPE_CHECKING, Union
 
 import numpy as np
 
@@ -51,6 +51,12 @@ def __init__(
         base = Circuit(contents, strategy=strategy)
         self._moments = tuple(base.moments)
 
+    @classmethod
+    def _from_moments(cls, moments: Iterable['cirq.Moment']) -> 'FrozenCircuit':
+        new_circuit = FrozenCircuit()
+        new_circuit._moments = tuple(moments)
+        return new_circuit
+
     @property
     def moments(self) -> Sequence['cirq.Moment']:
         return self._moments
@@ -143,11 +149,6 @@ def __pow__(self, other) -> 'cirq.FrozenCircuit':
         except:
             return NotImplemented
 
-    def _with_sliced_moments(self, moments: Iterable['cirq.Moment']) -> 'FrozenCircuit':
-        new_circuit = FrozenCircuit()
-        new_circuit._moments = tuple(moments)
-        return new_circuit
-
     def _resolve_parameters_(
         self, resolver: 'cirq.ParamResolver', recursive: bool
     ) -> 'cirq.FrozenCircuit':

cirq-core/cirq/circuits/frozen_circuit_test.py

@@ -21,6 +21,23 @@
 import cirq
 
 
+def test_from_moments():
+    a, b, c, d = cirq.LineQubit.range(4)
+    assert cirq.FrozenCircuit.from_moments(
+        [cirq.X(a), cirq.Y(b)],
+        [cirq.X(c)],
+        [],
+        cirq.Z(d),
+        [cirq.measure(a, b, key='ab'), cirq.measure(c, d, key='cd')],
+    ) == cirq.FrozenCircuit(
+        cirq.Moment(cirq.X(a), cirq.Y(b)),
+        cirq.Moment(cirq.X(c)),
+        cirq.Moment(),
+        cirq.Moment(cirq.Z(d)),
+        cirq.Moment(cirq.measure(a, b, key='ab'), cirq.measure(c, d, key='cd')),
+    )
+
+
 def test_freeze_and_unfreeze():
     a, b = cirq.LineQubit.range(2)
     c = cirq.Circuit(cirq.X(a), cirq.H(b))