Inherit options

src/marshmallow/schema.py

@@ -90,7 +90,12 @@ class SchemaMeta(type):
     the Schema class's ``class Meta`` options.
     """
 
-    def __new__(mcs, name, bases, attrs):
+    def __new__(mcs, name, bases, attrs, combine_opts=False):
+        """
+        :param combine_opts: If true, create a new SchemaOpts and Meta subclass from both parents
+        """
+
+        # Collect fields and construct the class
         meta = attrs.get("Meta")
         ordered = getattr(meta, "ordered", False)
         if not ordered:
@@ -108,10 +113,20 @@ def __new__(mcs, name, bases, attrs):
         klass = super().__new__(mcs, name, bases, attrs)
         inherited_fields = _get_fields_by_mro(klass, base.FieldABC, ordered=ordered)
 
+        # Instantiate the options class using class Meta
         meta = klass.Meta
-        # Set klass.opts in __new__ rather than __init__ so that it is accessible in
-        # get_declared_fields
-        klass.opts = klass.OPTIONS_CLASS(meta, ordered=ordered)
+        if combine_opts and len(bases) > 1:
+            # Create a new options class that combines all the base classes
+            combined_opts = type(
+                name + "Opts", tuple({base.OPTIONS_CLASS for base in bases}), {}
+            )
+            # Instantiate the options class as we normally do
+            klass.opts = combined_opts(meta, ordered=ordered)
+        else:
+            # Set klass.opts in __new__ rather than __init__ so that it is accessible in
+            # get_declared_fields
+            klass.opts = klass.OPTIONS_CLASS(meta, ordered=ordered)
+
         # Add fields specified in the `include` class Meta option
         cls_fields += list(klass.opts.include.items())
 
@@ -146,7 +161,7 @@ def get_declared_fields(
         """
         return dict_cls(inherited_fields + cls_fields)
 
-    def __init__(cls, name, bases, attrs):
+    def __init__(cls, name, bases, attrs, combine_opts=False):
         super().__init__(name, bases, attrs)
         if name and cls.opts.register:
             class_registry.register(name, cls)

tests/test_schema.py

@@ -18,6 +18,7 @@
     INCLUDE,
     RAISE,
     class_registry,
+    SchemaOpts,
 )
 from marshmallow.exceptions import (
     ValidationError,
@@ -2766,3 +2767,79 @@ class Meta:
         dumped = OSchema().dump({"foo": 42, "bar": 24})
         assert isinstance(dumped, OrderedDict)
         assert "bar" not in dumped
+
+
+class TestCombineOpts:
+    def test_different_opts(self):
+        """
+        Create two options classes, each belonging to a schema, then combine the two
+        schemas using inheritance. When combine_opts == True, we should have both options
+        being stored in self.opts. Otherwise, we should only get options defined in the
+        first options class stored in self.opts.
+        """
+
+        class Opts1(SchemaOpts):
+            def __init__(self, meta, **kwargs):
+                super().__init__(meta, **kwargs)
+                self.opt_1 = getattr(meta, "opt_1", None)
+
+        class Schema1(Schema):
+            OPTIONS_CLASS = Opts1
+            Field1 = fields.String()
+
+        class Opts2(SchemaOpts):
+            def __init__(self, meta, **kwargs):
+                super().__init__(meta, **kwargs)
+                self.opt_2 = getattr(meta, "opt_2", None)
+
+        class Schema2(Schema):
+            OPTIONS_CLASS = Opts2
+            Field2 = fields.String()
+
+        class NotCombined(Schema1, Schema2, combine_opts=False):
+            class Meta:
+                opt_1 = True
+                opt_2 = True
+
+        class Combined(Schema1, Schema2, combine_opts=True):
+            class Meta:
+                opt_1 = True
+                opt_2 = True
+
+        # Without using combine_opts, we should only get options defined by the first
+        # options class
+        assert NotCombined().opts.opt_1
+        assert not hasattr(NotCombined().opts, "opt_2")
+
+        # However when we use combine_opts=True, we should get options defined by both
+        # options classes
+        assert Combined().opts.opt_1
+        assert Combined().opts.opt_2
+
+    def test_same_opts(self):
+        """
+        Ensure this behaviour still works if the two parent schemas have the same opts
+        class
+        """
+
+        class Opts1(SchemaOpts):
+            def __init__(self, meta, **kwargs):
+                super().__init__(meta, **kwargs)
+                self.opt_1 = getattr(meta, "opt_1", None)
+
+        class Schema1(Schema):
+            OPTIONS_CLASS = Opts1
+            Field1 = fields.String()
+
+        class Schema2(Schema):
+            OPTIONS_CLASS = Opts1
+            Field2 = fields.String()
+
+        class Combined(Schema1, Schema2, combine_opts=True):
+            class Meta:
+                opt_1 = True
+                opt_2 = True
+
+        # However when we use combine_opts=True, we should get options defined by both
+        # options classes
+        assert Combined().opts.opt_1