Expand SWEEP_LIKE to work for {'t': [0, 1, 3]} (#2786)

Fixes: #2698

cirq/study/sweepable.py

@@ -15,6 +15,7 @@
 """Defines which types are Sweepable."""
 
 from typing import Dict, Iterable, Iterator, List, Union, cast
+import itertools
 
 from cirq._doc import document
 from cirq.study.resolver import ParamResolver, ParamResolverOrSimilarType
@@ -55,7 +56,22 @@ def to_sweeps(sweepable: Sweepable) -> List[Sweep]:
     if isinstance(sweepable, Sweep):
         return [sweepable]
     if isinstance(sweepable, dict):
-        return [_resolver_to_sweep(ParamResolver(cast(Dict, sweepable)))]
+        # change dictionary of lists to list of dictionaries
+        # of single values using Cartesian product.
+        newsweepable = {}
+        for key, value in sweepable.items():
+            if isinstance(value, Iterable):
+                newsweepable[key] = value
+            else:
+                newsweepable[key] = [value]
+        expandsweepable = [
+            dict(zip(newsweepable.keys(), v))
+            for v in itertools.product(*newsweepable.values())
+        ]
+        return [
+            _resolver_to_sweep(ParamResolver(cast(Dict, dictitem)))
+            for dictitem in expandsweepable
+        ]
     if isinstance(sweepable, Iterable) and not isinstance(sweepable, str):
         return [
             sweep for item in sweepable for sweep in to_sweeps(

cirq/study/sweepable_test.py

@@ -20,7 +20,6 @@
 
 import cirq
 
-
 def test_to_resolvers_none():
     assert list(cirq.to_resolvers(None)) == [cirq.ParamResolver({})]
 
@@ -83,6 +82,18 @@ def test_to_sweeps_iterable_sweeps():
     assert cirq.study.to_sweeps(sweeps) == sweeps
 
 
+def test_to_sweeps_dictionary_of_list():
+    assert cirq.study.to_sweeps({'t': [0, 2, 3]}) == \
+        cirq.study.to_sweeps([{'t': 0}, {'t': 2}, {'t': 3}])
+    assert cirq.study.to_sweeps({'t': [0, 1], 's': [2, 3], 'r': 4}) == \
+        cirq.study.to_sweeps([
+            {'t': 0, 's': 2, 'r': 4},
+            {'t': 0, 's': 3, 'r': 4},
+            {'t': 1, 's': 2, 'r': 4},
+            {'t': 1, 's': 3, 'r': 4},
+        ])
+
+
 def test_to_sweeps_invalid():
     with pytest.raises(TypeError, match='Unrecognized sweepable'):
         cirq.study.to_sweeps('nope')