[discuss] Validation behavior during deserialization vs. serialization

Is it ok that required fields doesn’t work in load() method?

From quickstart example:

class UserSchema(Schema):
    name = fields.String(required=True)
    email = fields.Email()

user = {'name': None, 'email': '[e-mail]'}
data, errors = UserSchema().dump(user)
errors  # {'name': 'Missing data for required field.'}

user = {'name': None, 'email': '[e-mail]'}
data, errors = UserSchema().load(user)
errors  # {}

I thought that load() method is used for loading model objects from input data and SHOULD support required fields. On the contrary, method dump() is used to serialize inner data and not requires validation at all. Whether I understand everything correctly?

[sloria]

As discussed in the above thread, defaults for deserialization can be defined in make_object or in a preprocessor. For example, if you want the deserialization defaults to be the same as serialization defaults, you could do the following:

from marshmallow import fields, Schema
from marshmallow.validate import Range

class GETSchema(Schema):

    page = fields.Integer(default=1)
    per_page = fields.Integer(default=10, validate=Range(min=10, max=50))
    order_by = fields.Select(['id', 'name', 'priority'], default='priority')
    sort = fields.Select(['asc', 'desc'], default='desc')

    def make_object(schema, in_data):
        for name, field in schema.fields.items():
            if name not in in_data:
                in_data[name] = field.default
        return in_data

schema = GETSchema()
schema.load({}).data  # {'order_by': 'priority', 'per_page': 10, 'sort': 'asc', 'page': 1}

In lieu of the missing parameter discussed in previous comments, you can take advantage of the fact that extra kwargs passed to fields are stored in each field’s metadata attribute. This would allow you to have different defaults between serialization/deserialization.

from marshmallow import fields, Schema
from marshmallow.validate import Range

class GETSchema(Schema):

    page = fields.Integer(default=1, missing='null')
    per_page = fields.Integer(default=10, missing='null', validate=Range(min=10, max=50))
    order_by = fields.Select(['id', 'name', 'priority'], default='priority', missing='id')
    sort = fields.Select(['asc', 'desc'], default='desc', missing='asc')

    def make_object(schema, in_data):
        for name, field in schema.fields.items():
            if name not in in_data and field.metadata.get('missing'):
                in_data[name] = field.metadata['missing']
        return in_data

schema = GETSchema()
schema.load({}).data  # {'order_by': 'id', 'per_page': 'null', 'sort': 'asc', 'page': 'null'}

Yes, I am aware that these workarounds are just that: hacky workarounds. I will open up an issue to reopen discussion of a built-in missing param.

EDIT: Fix first example

[andrewbaxter]

Yeah, thanks, I’m afraid I hadn’t noticed that. #189 still is a problem, but I think that’s not by design so I’ll go back to that issue. I’ll open another issue regarding the docs.