get_field_def: accept FieldNode instead of field name

Replicates graphql/graphql-js@6fd5607

Author: Cito

src/graphql/execution/execute.py

@@ -590,10 +590,7 @@ def resolve_field(
         resolve function, then calls complete_value to await coroutine objects,
         serialize scalars, or execute the sub-selection-set for objects.
         """
-        field_node = field_nodes[0]
-        field_name = field_node.name.value
-
-        field_def = get_field_def(self.schema, parent_type, field_name)
+        field_def = get_field_def(self.schema, parent_type, field_nodes[0])
         if not field_def:
             return Undefined
 
@@ -1191,7 +1188,7 @@ def assert_valid_execution_arguments(
 
 
 def get_field_def(
-    schema: GraphQLSchema, parent_type: GraphQLObjectType, field_name: str
+    schema: GraphQLSchema, parent_type: GraphQLObjectType, field_node: FieldNode
 ) -> GraphQLField:
     """Get field definition.
 
@@ -1204,6 +1201,8 @@ def get_field_def(
 
     For internal use only.
     """
+    field_name = field_node.name.value
+
     if field_name == "__schema" and schema.query_type == parent_type:
         return SchemaMetaFieldDef
     elif field_name == "__type" and schema.query_type == parent_type:

src/graphql/subscription/subscribe.py

@@ -165,10 +165,10 @@ async def execute_subscription(context: ExecutionContext) -> AsyncIterable[Any]:
     type_ = get_operation_root_type(schema, context.operation)
     fields = context.collect_fields(type_, context.operation.selection_set, {}, set())
     response_name, field_nodes = next(iter(fields.items()))
-    field_name = field_nodes[0].name.value
-    field_def = get_field_def(schema, type_, field_name)
+    field_def = get_field_def(schema, type_, field_nodes[0])
 
     if not field_def:
+        field_name = field_nodes[0].name.value
         raise GraphQLError(
             f"The subscription field '{field_name}' is not defined.", field_nodes
         )