Marshmallow: Self-referencing schema - Post validate on root only

[ralbertazzi]

[marshmallow v3]

Given a self-referencing schema (i.e. a MySchema where some fields are Nested('self'), I'd like to make a post_load validation step before returning my nested structure. Problem is: I'd like to perform this final validation step only on the root object and not in all the other nodes.
Do you know an easy solution to achieve this? Thanks!

[deckar01]

I would have assumed you could subclass the schema and declare the @post_load hook there, but self resolves to the subclass.

from marshmallow import fields, Schema, post_load


class MySchema(Schema):
    foo = fields.Nested('self', many=True)

class RootMySchema(MySchema):
    @post_load
    def bar(self, data):
        data['root'] = True
        return data

schema = RootMySchema()
obj = schema.load({'foo': [{'foo': []}]})
# {'foo': [{'foo': [], 'root': True}], 'root': True}

Using a string reference instead of "self" does not seem to work either. If you are using python 3 you can reference the class inside itself.

from marshmallow import fields, Schema, post_load


class MySchema(Schema):
    foo = fields.Nested(MySchema, many=True)

class RootMySchema(MySchema):
    @post_load
    def bar(self, data):
        data['root'] = True
        return data

schema = RootMySchema()
obj = schema.load({'foo': [{'foo': []}]})
# {'foo': [{'foo': []}], 'root': True}

If you are using python 2 you may have to override the recursive field on the top level schema as a work around.

from marshmallow import fields, Schema, post_load


class MySchema(Schema):
    foo = fields.Nested('MySchema', many=True)

class RootMySchema(MySchema):
    foo = fields.Nested(MySchema, many=True)

    @post_load
    def bar(self, data):
        data['root'] = True
        return data

schema = RootMySchema()
obj = schema.load({'foo': [{'foo': []}]})
# {'foo': [{'foo': []}], 'root': True}

Edit: Fixed missing @post_load.

[lafrech]

I think there is a @post_load missing in the Python 3 example.

(Since this is MA3) I really hope the OP is not using Python 2.