Improve "required" validation; allow_none and allow_blank

Currently, setting required=True on Field only returns an error if the field’s value is missing from the input data.

class UserSchema(Schema):
    name = fields.Str(required=True)

s = UserSchema()
s.validate({})  # {'name': ['Missing data for required field.']}
s.validate({'name': None})  #  No errors
s.validate({'name': ''})  # No errors

This may not meet the typical use case, where you want an error returned for None and the empty string (with the option to allow these values).

Solution

By default, return an error if None or '' are passed to a required field. Add allow_none and allow_blank options to allow these values to pass validation.

UserSchema().validate({'name': None})  #  {'name': ['Missing data for required field.']}

class LenientSchema(Schema):
    name = fields.Str(required=True, allow_none=True)

LenientSchema().validate({'name': None})  # No errors

Note: This idea was yoinked from Django Rest Framework. =)

[deckar01]

@loic001 It is as standard marshalling design to load unstructured data and dump structured data. The schema should be designed to match the data structure you want your application to work with. Any data that does not comply with that structure needs to be loaded. Any data that does comply with that structure can be dumped.

[sloria]

@vesauimonen An update on this issue: I’ve added the allow_none parameter to field classes, which makes validation of None consistent across fields.

With a slight modification of your code above, validation works as expected.

from marshmallow import fields, Schema

class User(object):
    def __init__(self, name, activated_at=None):
        self.name = name
        self.activated_at = activated_at


class UserSchema(Schema):
    name = fields.String(required=True)
    activated_at = fields.DateTime(allow_none=True)


user = User(name='John')

s = UserSchema()

serialized_result, errors = s.dump(user)
# no errors
assert errors == {}

deserialized_result, errors = s.load(serialized_result) 
# no errors
assert errors == {}