Simple implementation of DSL inline fragments

gql/dsl.py

@@ -17,6 +17,7 @@
     GraphQLObjectType,
     GraphQLSchema,
     GraphQLWrappingType,
+    InlineFragmentNode,
     ListTypeNode,
     ListValueNode,
     NamedTypeNode,
@@ -407,6 +408,10 @@ class DSLField:
     method.
     """
 
+    _type: Union[GraphQLObjectType, GraphQLInterfaceType]
+    ast_field: FieldNode
+    field: GraphQLField
+
     def __init__(
         self,
         name: str,
@@ -423,11 +428,9 @@ def __init__(
         :param graphql_type: the GraphQL type definition from the schema
         :param graphql_field: the GraphQL field definition from the schema
         """
-        self._type: Union[GraphQLObjectType, GraphQLInterfaceType] = graphql_type
-        self.field: GraphQLField = graphql_field
-        self.ast_field: FieldNode = FieldNode(
-            name=NameNode(value=name), arguments=FrozenList()
-        )
+        self._type = graphql_type
+        self.field = graphql_field
+        self.ast_field = FieldNode(name=NameNode(value=name), arguments=FrozenList())
         log.debug(f"Creating {self!r}")
 
     @staticmethod
@@ -585,7 +588,25 @@ def __str__(self) -> str:
         return print_ast(self.ast_field)
 
     def __repr__(self) -> str:
-        return (
-            f"<{self.__class__.__name__} {self._type.name}"
-            f"::{self.ast_field.name.value}>"
+        name = self._type.name
+        try:
+            name += f"::{self.ast_field.name.value}"
+        except AttributeError:
+            pass
+        return f"<{self.__class__.__name__} {name}>"
+
+
+class DSLFragment(DSLField):
+    def __init__(
+        self, type_condition: Optional[DSLType] = None,
+    ):
+        self.ast_field = InlineFragmentNode()  # type: ignore
+        if type_condition:
+            self.on(type_condition)
+
+    def on(self, type_condition: DSLType):
+        self._type = type_condition._type
+        self.ast_field.type_condition = NamedTypeNode(  # type: ignore
+            name=NameNode(value=self._type.name)
         )
+        return self

tests/starwars/test_dsl.py

@@ -15,6 +15,7 @@
 
 from gql import Client
 from gql.dsl import (
+    DSLFragment,
     DSLMutation,
     DSLQuery,
     DSLSchema,
@@ -416,6 +417,24 @@ def test_multiple_operations(ds):
     )
 
 
+def test_inline_fragments(ds):
+    query = """hero(episode: JEDI) {
+  name
+  ... on Droid {
+    primaryFunction
+  }
+  ... on Human {
+    homePlanet
+  }
+}"""
+    query_dsl = ds.Query.hero.args(episode=6).select(
+        ds.Character.name,
+        DSLFragment().on(ds.Droid).select(ds.Droid.primaryFunction),
+        DSLFragment().on(ds.Human).select(ds.Human.homePlanet),
+    )
+    assert query == str(query_dsl)
+
+
 def test_dsl_query_all_fields_should_be_instances_of_DSLField():
     with pytest.raises(
         TypeError, match="fields must be instances of DSLField. Received type:"