DSL Allow to set a default value for variables

docs/advanced/dsl_module.rst

@@ -206,6 +206,35 @@ will generate a query equivalent to::
       }
     }
 
+Variable arguments with a default value
+"""""""""""""""""""""""""""""""""""""""
+
+If you want to provide a **default value** for your variable, you can use
+the :code:`default` method on a variable.
+
+The following code:
+
+.. code-block:: python
+
+    var = DSLVariableDefinitions()
+    op = DSLMutation(
+        ds.Mutation.createReview.args(
+            review=var.review.default({"stars": 5, "commentary": "Wow!"}),
+            episode=var.episode,
+        ).select(ds.Review.stars, ds.Review.commentary)
+    )
+    op.variable_definitions = var
+    query = dsl_gql(op)
+
+will generate a query equivalent to::
+
+    mutation ($review: ReviewInput = {stars: 5, commentary: "Wow!"}, $episode: Episode) {
+      createReview(review: $review, episode: $episode) {
+        stars
+        commentary
+      }
+    }
+
 Subscriptions
 ^^^^^^^^^^^^^
 

gql/dsl.py

@@ -18,6 +18,7 @@
     FragmentDefinitionNode,
     FragmentSpreadNode,
     GraphQLArgument,
+    GraphQLEnumType,
     GraphQLError,
     GraphQLField,
     GraphQLID,
@@ -28,9 +29,9 @@
     GraphQLNamedType,
     GraphQLNonNull,
     GraphQLObjectType,
+    GraphQLScalarType,
     GraphQLSchema,
     GraphQLString,
-    GraphQLWrappingType,
     InlineFragmentNode,
     IntValueNode,
     ListTypeNode,
@@ -50,7 +51,6 @@
     ValueNode,
     VariableDefinitionNode,
     VariableNode,
-    assert_named_type,
     get_named_type,
     introspection_types,
     is_enum_type,
@@ -134,7 +134,7 @@ def ast_from_value(value: Any, type_: GraphQLInputType) -> Optional[ValueNode]:
     of if we receive a Null value for a Non-Null type.
     """
     if isinstance(value, DSLVariable):
-        return value.set_type(type_).ast_variable
+        return value.set_type(type_).ast_variable_name
 
     if is_non_null_type(type_):
         type_ = cast(GraphQLNonNull, type_)
@@ -529,26 +529,33 @@ class DSLVariable:
 
     def __init__(self, name: str):
         """:meta private:"""
-        self.type: Optional[TypeNode] = None
         self.name = name
-        self.ast_variable = VariableNode(name=NameNode(value=self.name))
+        self.ast_variable_type: Optional[TypeNode] = None
+        self.ast_variable_name = VariableNode(name=NameNode(value=self.name))
+        self.default_value = None
+        self.type: Optional[GraphQLInputType] = None
 
-    def to_ast_type(
-        self, type_: Union[GraphQLWrappingType, GraphQLNamedType]
-    ) -> TypeNode:
+    def to_ast_type(self, type_: GraphQLInputType) -> TypeNode:
         if is_wrapping_type(type_):
             if isinstance(type_, GraphQLList):
                 return ListTypeNode(type=self.to_ast_type(type_.of_type))
+
             elif isinstance(type_, GraphQLNonNull):
                 return NonNullTypeNode(type=self.to_ast_type(type_.of_type))
 
-        type_ = assert_named_type(type_)
+        assert isinstance(
+            type_, (GraphQLScalarType, GraphQLEnumType, GraphQLInputObjectType)
+        )
+
         return NamedTypeNode(name=NameNode(value=type_.name))
 
-    def set_type(
-        self, type_: Union[GraphQLWrappingType, GraphQLNamedType]
-    ) -> "DSLVariable":
-        self.type = self.to_ast_type(type_)
+    def set_type(self, type_: GraphQLInputType) -> "DSLVariable":
+        self.type = type_
+        self.ast_variable_type = self.to_ast_type(type_)
+        return self
+
+    def default(self, default_value: Any) -> "DSLVariable":
+        self.default_value = default_value
         return self
 
 
@@ -581,9 +588,11 @@ def get_ast_definitions(self) -> Tuple[VariableDefinitionNode, ...]:
         """
         return tuple(
             VariableDefinitionNode(
-                type=var.type,
-                variable=var.ast_variable,
-                default_value=None,
+                type=var.ast_variable_type,
+                variable=var.ast_variable_name,
+                default_value=None
+                if var.default_value is None
+                else ast_from_value(var.default_value, var.type),
             )
             for var in self.variables.values()
             if var.type is not None  # only variables used

tests/starwars/test_dsl.py

@@ -111,11 +111,11 @@ def test_ast_from_serialized_value_untyped_typeerror():
 
 
 def test_variable_to_ast_type_passing_wrapping_type():
-    wrapping_type = GraphQLNonNull(GraphQLList(StarWarsSchema.get_type("Droid")))
-    variable = DSLVariable("droids")
+    wrapping_type = GraphQLNonNull(GraphQLList(StarWarsSchema.get_type("ReviewInput")))
+    variable = DSLVariable("review_input")
     ast = variable.to_ast_type(wrapping_type)
     assert ast == NonNullTypeNode(
-        type=ListTypeNode(type=NamedTypeNode(name=NameNode(value="Droid")))
+        type=ListTypeNode(type=NamedTypeNode(name=NameNode(value="ReviewInput")))
     )
 
 
@@ -170,6 +170,50 @@ def test_add_variable_definitions(ds):
     )
 
 
+def test_add_variable_definitions_with_default_value_enum(ds):
+    var = DSLVariableDefinitions()
+    op = DSLMutation(
+        ds.Mutation.createReview.args(
+            review=var.review, episode=var.episode.default(4)
+        ).select(ds.Review.stars, ds.Review.commentary)
+    )
+    op.variable_definitions = var
+    query = dsl_gql(op)
+
+    assert (
+        print_ast(query)
+        == """mutation ($review: ReviewInput, $episode: Episode = NEWHOPE) {
+  createReview(review: $review, episode: $episode) {
+    stars
+    commentary
+  }
+}"""
+    )
+
+
+def test_add_variable_definitions_with_default_value_input_object(ds):
+    var = DSLVariableDefinitions()
+    op = DSLMutation(
+        ds.Mutation.createReview.args(
+            review=var.review.default({"stars": 5, "commentary": "Wow!"}),
+            episode=var.episode,
+        ).select(ds.Review.stars, ds.Review.commentary)
+    )
+    op.variable_definitions = var
+    query = dsl_gql(op)
+
+    assert (
+        print_ast(query)
+        == """
+mutation ($review: ReviewInput = {stars: 5, commentary: "Wow!"}, $episode: Episode) {
+  createReview(review: $review, episode: $episode) {
+    stars
+    commentary
+  }
+}""".strip()
+    )
+
+
 def test_add_variable_definitions_in_input_object(ds):
     var = DSLVariableDefinitions()
     op = DSLMutation(