Add q helper function for constructing common qubit types.

cirq-core/cirq/__init__.py

@@ -291,6 +291,7 @@
     ProjectorString,
     ProjectorSum,
     RandomGateChannel,
+    q,
     qft,
     Qid,
     QuantumFourierTransformGate,

cirq-core/cirq/ops/__init__.py

@@ -308,3 +308,5 @@
 )
 
 from cirq.ops.state_preparation_channel import StatePreparationChannel
+
+from cirq.ops.util import q

cirq-core/cirq/ops/util.py

@@ -0,0 +1,68 @@
+# Copyright 2022 The Cirq Developers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from typing import Optional, overload, TYPE_CHECKING, Union
+
+if TYPE_CHECKING:
+    import cirq
+
+
+@overload
+def q(x: int) -> 'cirq.LineQubit':
+    ...
+
+
+@overload
+def q(x: int, y: int) -> 'cirq.GridQubit':
+    ...
+
+
+@overload
+def q(x: str) -> 'cirq.NamedQubit':
+    ...
+
+
+def q(
+    x: Union[int, str], y: Optional[int] = None
+) -> Union['cirq.LineQubit', 'cirq.GridQubit', 'cirq.NamedQubit']:
+    """Constructs a qubit id of the appropriate type based on args.
+
+    This is shorthand for constructing qubit ids of common types:
+    >>> cirq.q(1) == cirq.LineQubit(1)
+    >>> cirq.q(1, 2) == cirq.GridQubit(1, 2)
+    >>> cirq.q("foo") == cirq.NamedQubit("foo")
+
+    Args:
+        x: First arg.
+        y: Second arg.
+
+    Returns:
+        cirq.LineQubit if called with one integer arg.
+        cirq.GridQubit if called with two integer args.
+        cirq.NamedQubit if called with one string arg.
+
+    Raises:
+        ValueError: if called with invalid arguments.
+    """
+    import cirq  # avoid circular import
+
+    if y is None:
+        if isinstance(x, int):
+            return cirq.LineQubit(x)
+        elif isinstance(x, str):
+            return cirq.NamedQubit(x)
+    else:
+        if isinstance(x, int) and isinstance(y, int):
+            return cirq.GridQubit(x, y)
+    raise ValueError(f"Could not construct qubit: args={(x, y)}")

cirq-core/cirq/ops/util_test.py

@@ -0,0 +1,31 @@
+# Copyright 2022 The Cirq Developers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import pytest
+
+import cirq
+
+
+def test_q() -> None:
+    assert cirq.q(0) == cirq.LineQubit(0)
+    assert cirq.q(1, 2) == cirq.GridQubit(1, 2)
+    assert cirq.q("foo") == cirq.NamedQubit("foo")
+
+
+def test_q_invalid() -> None:
+    # Ignore static type errors so we can test runtime typechecks.
+    with pytest.raises(ValueError):
+        cirq.q([1, 2, 3])  # type: ignore[call-overload]
+    with pytest.raises(ValueError):
+        cirq.q(1, "foo")  # type: ignore[call-overload]