Workaround mypy bug by moving Sweepable into its own file (#388)

Author: Strilanc

cirq/study/__init__.py

@@ -18,9 +18,11 @@
     ParamResolver,
 )
 from cirq.study.study import (
-    Sweepable,
     TrialResult,
 )
+from cirq.study.sweepable import (
+    Sweepable,
+)
 from cirq.study.sweeps import (
     Points,
     Sweep,

cirq/study/study.py

@@ -12,13 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-"""Defines a study.
-"""
-
-from typing import Iterable, Union
-
-from cirq.study.resolver import ParamResolver
-from cirq.study.sweeps import Sweep
+"""Defines a study."""
 
 
 class TrialResultMeta(type):
@@ -49,7 +43,3 @@ class TrialResult(metaclass=TrialResultMeta):
             the first index running over the repetitions, and the second index
             running over the qubits for the corresponding measurements.
     """
-
-
-Sweepable = Union[
-    ParamResolver, Iterable[ParamResolver], Sweep, Iterable[Sweep]]

cirq/study/sweepable.py

@@ -0,0 +1,24 @@
+# Copyright 2018 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.
+
+"""Defines which types are Sweepable."""
+
+from typing import Iterable, Union
+
+from cirq.study.resolver import ParamResolver
+from cirq.study.sweeps import Sweep
+
+
+Sweepable = Union[
+    ParamResolver, Iterable[ParamResolver], Sweep, Iterable[Sweep]]