Optimize parallel execution when it's only one task

See discussion in issue #190.

Also, run coverage in Python 3.10, since running in Python 3.11
reports false negatives in test_lists (probably bug in coverage).

Author: Cito

src/graphql/__init__.py

@@ -313,7 +313,7 @@
     # Validate GraphQL schema.
     validate_schema,
     assert_valid_schema,
-    #  Uphold the spec rules about naming
+    # Uphold the spec rules about naming
     assert_name,
     assert_enum_value_name,
     # Types

src/graphql/execution/execute.py

@@ -441,7 +441,7 @@ def execute_fields(
                 if is_awaitable(result):
                     append_awaitable(response_name)
 
-        #  If there are no coroutines, we can just return the object
+        # If there are no coroutines, we can just return the object.
         if not awaitable_fields:
             return results
 
@@ -450,12 +450,17 @@ def execute_fields(
         # will yield this same map, but with any coroutines awaited in parallel and
         # replaced with the values they yielded.
         async def get_results() -> Dict[str, Any]:
-            results.update(
-                zip(
-                    awaitable_fields,
-                    await gather(*(results[field] for field in awaitable_fields)),
+            if len(awaitable_fields) == 1:
+                # If there is only one field, avoid the overhead of parallelization.
+                field = awaitable_fields[0]
+                results[field] = await results[field]
+            else:
+                results.update(
+                    zip(
+                        awaitable_fields,
+                        await gather(*(results[field] for field in awaitable_fields)),
+                    )
                 )
-            )
             return results
 
         return get_results()
@@ -758,13 +763,18 @@ async def await_completed(item: Any, item_path: Path) -> Any:
 
         # noinspection PyShadowingNames
         async def get_completed_results() -> List[Any]:
-            for index, result in zip(
-                awaitable_indices,
-                await gather(
-                    *(completed_results[index] for index in awaitable_indices)
-                ),
-            ):
-                completed_results[index] = result
+            if len(awaitable_indices) == 1:
+                # If there is only one index, avoid the overhead of parallelization.
+                index = awaitable_indices[0]
+                completed_results[index] = await completed_results[index]
+            else:
+                for index, result in zip(
+                    awaitable_indices,
+                    await gather(
+                        *(completed_results[index] for index in awaitable_indices)
+                    ),
+                ):
+                    completed_results[index] = result
             return completed_results
 
         return get_completed_results()
@@ -907,7 +917,7 @@ def complete_object_value(
 
         # If there is an `is_type_of()` predicate function, call it with the current
         # result. If `is_type_of()` returns False, then raise an error rather than
-        #  continuing execution.
+        # continuing execution.
         if return_type.is_type_of:
             is_type_of = return_type.is_type_of(result, info)
 
@@ -943,7 +953,7 @@ def collect_subfields(
         # We cannot use the field_nodes themselves as key for the cache, since they
         # are not hashable as a list. We also do not want to use the field_nodes
         # themselves (converted to a tuple) as keys, since hashing them is slow.
-        # Therefore we use the ids of the field_nodes as keys. Note that we do not
+        # Therefore, we use the ids of the field_nodes as keys. Note that we do not
         # use the id of the list, since we want to hit the cache for all lists of
         # the same nodes, not only for the same list of nodes. Also, the list id may
         # even be reused, in which case we would get wrong results from the cache.

tests/execution/test_lists.py

@@ -25,6 +25,21 @@ def _complete(list_field):
             Data(list_field),
         )
 
+    def accepts_a_list_as_a_list_value():
+        result = _complete([])
+        assert result == ({"listField": []}, None)
+        list_field = ["just an apple"]
+        result = _complete(list_field)
+        assert result == ({"listField": list_field}, None)
+        list_field = ["apple", "banana", "coconut"]
+        result = _complete(list_field)
+        assert result == ({"listField": list_field}, None)
+
+    def accepts_a_tuple_as_a_list_value():
+        list_field = ("apple", "banana", "coconut")
+        result = _complete(list_field)
+        assert result == ({"listField": list(list_field)}, None)
+
     def accepts_a_set_as_a_list_value():
         # Note that sets are not ordered in Python.
         list_field = {"apple", "banana", "coconut"}

tests/execution/test_parallel.py

@@ -32,6 +32,27 @@ async def wait(self) -> bool:
 
 
 def describe_parallel_execution():
+    @mark.asyncio
+    async def resolve_single_field():
+        # make sure that the special case of resolving a single field works
+        async def resolve(*_args):
+            return True
+
+        schema = GraphQLSchema(
+            GraphQLObjectType(
+                "Query",
+                {
+                    "foo": GraphQLField(GraphQLBoolean, resolve=resolve),
+                },
+            )
+        )
+
+        awaitable_result = execute(schema, parse("{foo}"))
+        assert isinstance(awaitable_result, Awaitable)
+        result = await awaitable_result
+
+        assert result == ({"foo": True}, None)
+
     @mark.asyncio
     async def resolve_fields_in_parallel():
         barrier = Barrier(2)
@@ -58,6 +79,25 @@ async def resolve(*_args):
 
         assert result == ({"foo": True, "bar": True}, None)
 
+    @mark.asyncio
+    async def resolve_single_element_list():
+        # make sure that the special case of resolving a single element list works
+        async def resolve(*_args):
+            return [True]
+
+        schema = GraphQLSchema(
+            GraphQLObjectType(
+                "Query",
+                {"foo": GraphQLField(GraphQLList(GraphQLBoolean), resolve=resolve)},
+            )
+        )
+
+        awaitable_result = execute(schema, parse("{foo}"))
+        assert isinstance(awaitable_result, Awaitable)
+        result = await awaitable_result
+
+        assert result == ({"foo": [True]}, None)
+
     @mark.asyncio
     async def resolve_list_in_parallel():
         barrier = Barrier(2)

tox.ini

@@ -62,5 +62,5 @@ deps =
 commands =
     # to also run the time-consuming tests: tox -e py310 -- --run-slow
     # to run the benchmarks: tox -e py310 -- -k benchmarks --benchmark-enable
-    py37,py38.py39,py310,pypy39: pytest tests {posargs}
-    py311: pytest tests {posargs: --cov-report=term-missing --cov=graphql --cov=tests --cov-fail-under=100}
+    py37,py38.py39,py311,pypy39: pytest tests {posargs}
+    py310: pytest tests {posargs: --cov-report=term-missing --cov=graphql --cov=tests --cov-fail-under=100}