GraphQLScalarType: make 'serialize' optional with 'identity_func' as default

It's a logical step since this is what 'build_schema', 'extend_schema' and
'build_client_schema' are doing anyway.

Replicates graphql/graphql-js@71d96d7

Author: Cito

graphql/type/definition.py

@@ -307,18 +307,18 @@ def serialize_odd(value):
     # Serializes an internal value to include in a response.
     serialize: GraphQLScalarSerializer
     #  Parses an externally provided value to use as an input.
-    parseValue: GraphQLScalarValueParser
+    parse_value: GraphQLScalarValueParser
     # Parses an externally provided literal value to use as an input.
     # Takes a dictionary of variables as an optional second argument.
-    parseLiteral: GraphQLScalarLiteralParser
+    parse_literal: GraphQLScalarLiteralParser
 
     ast_node: Optional[ScalarTypeDefinitionNode]
     extension_ast_nodes: Optional[Tuple[ScalarTypeExtensionNode]]
 
     def __init__(
         self,
         name: str,
-        serialize: GraphQLScalarSerializer,
+        serialize: GraphQLScalarSerializer = None,
         description: str = None,
         parse_value: GraphQLScalarValueParser = None,
         parse_literal: GraphQLScalarLiteralParser = None,
@@ -331,7 +331,7 @@ def __init__(
             ast_node=ast_node,
             extension_ast_nodes=extension_ast_nodes,
         )
-        if not callable(serialize):
+        if serialize is not None and not callable(serialize):
             raise TypeError(
                 f"{name} must provide 'serialize' function."
                 " If this custom Scalar is also used as an input type,"
@@ -352,9 +352,9 @@ def __init__(
             raise TypeError(
                 f"{name} extension AST nodes must be ScalarTypeExtensionNode."
             )
-        self.serialize = serialize  # type: ignore
-        self.parse_value = parse_value or identity_func
-        self.parse_literal = parse_literal or value_from_ast_untyped
+        self.serialize = serialize or identity_func  # type: ignore
+        self.parse_value = parse_value or identity_func  # type: ignore
+        self.parse_literal = parse_literal or value_from_ast_untyped  # type: ignore
 
     def __repr__(self):
         return f"<{self.__class__.__name__} {self.name!r}>"

graphql/utilities/build_ast_schema.py

@@ -24,7 +24,7 @@
     parse,
     Node,
 )
-from ..pyutils import identity_func, inspect
+from ..pyutils import inspect
 from ..type import (
     GraphQLArgument,
     GraphQLDeprecatedDirective,
@@ -385,7 +385,6 @@ def _make_scalar_def(ast_node: ScalarTypeDefinitionNode) -> GraphQLScalarType:
             name=ast_node.name.value,
             description=ast_node.description.value if ast_node.description else None,
             ast_node=ast_node,
-            serialize=identity_func,
         )
 
     def _make_input_object_def(

graphql/utilities/build_client_schema.py

@@ -3,7 +3,7 @@
 
 from ..error import INVALID
 from ..language import DirectiveLocation, parse_value
-from ..pyutils import identity_func, inspect
+from ..pyutils import inspect
 from ..type import (
     GraphQLArgument,
     GraphQLDirective,
@@ -126,7 +126,6 @@ def build_scalar_def(scalar_introspection: Dict) -> GraphQLScalarType:
         return GraphQLScalarType(
             name=scalar_introspection["name"],
             description=scalar_introspection.get("description"),
-            serialize=identity_func,
         )
 
     def build_object_def(object_introspection: Dict) -> GraphQLObjectType:

tests/type/test_definition.py

@@ -22,7 +22,7 @@
 )
 from graphql.utilities import value_from_ast_untyped
 
-ScalarType = GraphQLScalarType("Scalar", serialize=lambda: None)
+ScalarType = GraphQLScalarType("Scalar")
 ObjectType = GraphQLObjectType("Object", {})
 InterfaceType = GraphQLInterfaceType("Interface")
 UnionType = GraphQLUnionType("Union", [ObjectType], resolve_type=lambda: None)
@@ -37,39 +37,20 @@
 
 def describe_type_system_scalars():
     def accepts_a_scalar_type_defining_serialize():
-        assert GraphQLScalarType("SomeScalar", lambda: None)
+        assert GraphQLScalarType("SomeScalar")
 
     def accepts_a_scalar_type_defining_parse_value_and_parse_literal():
         assert GraphQLScalarType(
-            "SomeScalar",
-            serialize=lambda: None,
-            parse_value=lambda: None,
-            parse_literal=lambda: None,
+            "SomeScalar", parse_value=lambda: None, parse_literal=lambda: None
         )
 
     def provides_default_methods_if_omitted():
-        scalar = GraphQLScalarType("Foo", lambda: None)
+        scalar = GraphQLScalarType("Foo")
 
+        assert scalar.serialize is identity_func
         assert scalar.parse_value is identity_func
         assert scalar.parse_literal is value_from_ast_untyped
 
-    def rejects_a_scalar_type_not_defining_serialize():
-        with raises(
-            TypeError, match="missing 1 required positional argument: 'serialize'"
-        ):
-            # noinspection PyArgumentList
-            GraphQLScalarType("SomeScalar")
-        with raises(TypeError) as exc_info:
-            # noinspection PyTypeChecker
-            GraphQLScalarType("SomeScalar", None)
-        msg = str(exc_info.value)
-        assert msg == (
-            "SomeScalar must provide 'serialize' function."
-            " If this custom Scalar is also used as an input type,"
-            " ensure 'parse_value' and 'parse_literal' functions"
-            " are also provided."
-        )
-
     def rejects_a_scalar_type_defining_serialize_with_incorrect_type():
         with raises(TypeError) as exc_info:
             # noinspection PyTypeChecker
@@ -84,7 +65,7 @@ def rejects_a_scalar_type_defining_serialize_with_incorrect_type():
 
     def rejects_a_scalar_type_defining_parse_value_but_not_parse_literal():
         with raises(TypeError) as exc_info:
-            GraphQLScalarType("SomeScalar", lambda: None, parse_value=lambda: None)
+            GraphQLScalarType("SomeScalar", parse_value=lambda: None)
         msg = str(exc_info.value)
         assert msg == (
             "SomeScalar must provide both"
@@ -93,7 +74,7 @@ def rejects_a_scalar_type_defining_parse_value_but_not_parse_literal():
 
     def rejects_a_scalar_type_defining_parse_literal_but_not_parse_value():
         with raises(TypeError) as exc_info:
-            GraphQLScalarType("SomeScalar", lambda: None, parse_literal=lambda: None)
+            GraphQLScalarType("SomeScalar", parse_literal=lambda: None)
         msg = str(exc_info.value)
         assert msg == (
             "SomeScalar must provide both"
@@ -103,9 +84,7 @@ def rejects_a_scalar_type_defining_parse_literal_but_not_parse_value():
     def rejects_a_scalar_type_incorrectly_defining_parse_literal_and_value():
         with raises(TypeError) as exc_info:
             # noinspection PyTypeChecker
-            GraphQLScalarType(
-                "SomeScalar", lambda: None, parse_value={}, parse_literal={}
-            )
+            GraphQLScalarType("SomeScalar", parse_value={}, parse_literal={})
         msg = str(exc_info.value)
         assert msg == (
             "SomeScalar must provide both"

tests/type/test_predicate.py

@@ -71,7 +71,7 @@
 UnionType = GraphQLUnionType("Union", types=[ObjectType])
 EnumType = GraphQLEnumType("Enum", values={"foo": {}})
 InputObjectType = GraphQLInputObjectType("InputObject", {})
-ScalarType = GraphQLScalarType("Scalar", lambda: None)
+ScalarType = GraphQLScalarType("Scalar")
 Directive = GraphQLDirective("Directive", [DirectiveLocation.QUERY])
 
 
@@ -573,7 +573,7 @@ def returns_false_for_spec_defined_scalar_type():
                 assert is_specified_directive(GraphQLString) is False
 
             def returns_false_for_scalar_type_named_like_specified_directive():
-                ScalarNamedLikeDirective = GraphQLScalarType("deprecated", lambda: None)
+                ScalarNamedLikeDirective = GraphQLScalarType("deprecated")
                 assert is_specified_directive(ScalarNamedLikeDirective) is False
 
             def returns_false_for_random_garbage():

tests/type/test_schema.py

@@ -209,7 +209,7 @@ def checks_the_configuration_for_mistakes():
 
     def describe_a_schema_must_contain_uniquely_named_types():
         def rejects_a_schema_which_redefines_a_built_in_type():
-            FakeString = GraphQLScalarType("String", serialize=lambda: None)
+            FakeString = GraphQLScalarType("String")
 
             QueryType = GraphQLObjectType(
                 "Query",

tests/type/test_validation.py

@@ -29,7 +29,7 @@
 from graphql.utilities import build_schema, extend_schema
 
 
-SomeScalarType = GraphQLScalarType(name="SomeScalar", serialize=lambda: None)
+SomeScalarType = GraphQLScalarType(name="SomeScalar")
 
 SomeInterfaceType = GraphQLInterfaceType(
     name="SomeInterface", fields=lambda: {"f": GraphQLField(SomeObjectType)}

tests/utilities/test_extend_schema.py

@@ -40,7 +40,7 @@
 
 # Test schema.
 
-SomeScalarType = GraphQLScalarType(name="SomeScalar", serialize=lambda x: x)
+SomeScalarType = GraphQLScalarType(name="SomeScalar")
 
 SomeInterfaceType = GraphQLInterfaceType(
     name="SomeInterface",

tests/utilities/test_schema_printer.py

@@ -410,7 +410,7 @@ def prints_input_type():
         )
 
     def prints_custom_scalar():
-        odd_type = GraphQLScalarType(name="Odd", serialize=lambda: None)
+        odd_type = GraphQLScalarType(name="Odd")
 
         schema = GraphQLSchema(types=[odd_type])
         output = print_for_test(schema)

tests/validation/harness.py

@@ -230,7 +230,6 @@ def raise_type_error(message):
 
 InvalidScalar = GraphQLScalarType(
     name="Invalid",
-    serialize=lambda value: value,
     parse_literal=lambda value_node: raise_type_error(
         f"Invalid scalar is always invalid: {print_ast(value_node)}"
     ),
@@ -239,7 +238,7 @@ def raise_type_error(message):
     ),
 )
 
-AnyScalar = GraphQLScalarType(name="Any", serialize=lambda value: value)
+AnyScalar = GraphQLScalarType(name="Any")
 
 QueryRoot = GraphQLObjectType(
     "QueryRoot",