Allow setting required=True on fields of type Optional (lovasoa#159)

* Allow setting `required=True` on fields of type `Optional`

* Brown bag...

I made a last-minute change to the new test, and didn't check that it
still ran.

Author: dairiki

marshmallow_dataclass/__init__.py

@@ -491,8 +491,9 @@ def _field_for_generic_type(
             if typing_inspect.is_optional_type(typ):
                 metadata["allow_none"] = metadata.get("allow_none", True)
                 metadata["dump_default"] = metadata.get("dump_default", None)
-                metadata["load_default"] = metadata.get("load_default", None)
-                metadata["required"] = False
+                if not metadata.get("required"):
+                    metadata["load_default"] = metadata.get("load_default", None)
+                metadata.setdefault("required", False)
             subtypes = [t for t in arguments if t is not NoneType]  # type: ignore
             if len(subtypes) == 1:
                 return field_for_schema(
@@ -549,7 +550,7 @@ def field_for_schema(
         if not metadata.get("required"):
             metadata.setdefault("load_default", default)
     else:
-        metadata.setdefault("required", True)
+        metadata.setdefault("required", not typing_inspect.is_optional_type(typ))
 
     # If the field was already defined by the user
     predefined_field = metadata.get("marshmallow_field")

tests/test_optional.py

@@ -37,3 +37,20 @@ class OptionalValueNotNone:
         self.assertEqual(
             exc_cm.exception.messages, {"value": ["Field may not be null."]}
         )
+
+    def test_required_optional_field(self):
+        @dataclass
+        class RequiredOptionalValue:
+            value: Optional[str] = field(default="default", metadata={"required": True})
+
+        schema = RequiredOptionalValue.Schema()
+
+        self.assertEqual(schema.load({"value": None}), RequiredOptionalValue(None))
+        self.assertEqual(
+            schema.load({"value": "hello"}), RequiredOptionalValue(value="hello")
+        )
+        with self.assertRaises(marshmallow.exceptions.ValidationError) as exc_cm:
+            schema.load({})
+        self.assertEqual(
+            exc_cm.exception.messages, {"value": ["Missing data for required field."]}
+        )