Allow request string to be passed in as the ast

This allows for parsing the ast beforehand doing some work specific to the query and then just passing that in to the normal `graphql` call rather than having to stitch everything together manually.

Author: markusdap

graphql/graphql.py

@@ -1,6 +1,7 @@
 from .execution import ExecutionResult, execute
 from .language.parser import parse
 from .language.source import Source
+from .language.ast import Document
 from .validation import validate
 
 
@@ -30,8 +31,11 @@ def graphql(schema, request_string='', root_value=None, context_value=None,
             variable_values=None, operation_name=None, executor=None,
             return_promise=False):
     try:
-        source = Source(request_string, 'GraphQL request')
-        ast = parse(source)
+        if isinstance(request_string, Document):
+            ast = request_string
+        else:
+            source = Source(request_string, 'GraphQL request')
+            ast = parse(source)
         validation_errors = validate(schema, ast)
         if validation_errors:
             return ExecutionResult(