colander workaround for the empty MappingSchema

colander_workaround_empty_mappingschema.py

#!/usr/bin/env python

from __future__ import print_function

import sys
import colander

from contextlib import contextmanager

person_1 = {
    'firstname': 'foo',
    'lastname': 'bar',
    'address': {}
}

person_2 = {
    'firstname': 'foobar',
    'lastname': 'barbaz',
    'address': {
        'city': 'Paris',
        'country': 'France',
        'zipcode': 75009
    }
}


class Address(colander.MappingSchema):
    city = colander.SchemaNode(colander.String())
    country = colander.SchemaNode(colander.String())
    zipcode = colander.SchemaNode(colander.Integer())


class Person(colander.Schema):
    lastname = colander.SchemaNode(colander.String())
    firstname = colander.SchemaNode(colander.String())
    address = Address(require=False, missing=colander.null)

    @contextmanager
    def _handle_empty_address(self, cstruct):
        """
        Skip it, just the time to validation
        And reinject it to the end of validating.
        """
        try:
            address = cstruct['address']
            is_empty = all((address[field] == colander.null) for field in address)
            if is_empty:
                cstruct['address'] = colander.null

            yield cstruct

        finally:
            cstruct['address'] = address

    def deserialize(self, cstruct=colander.null):
        with self._handle_empty_address(cstruct) as cstruct:
            cstruct = super(Person, self).deserialize(cstruct)

        return cstruct


def main():
    validator = Person()

    for person in (person_1, person_2):
        try:
            cstruct = validator.deserialize(person)
        except colander.Invalid as err:
            print(err, file=sys.stderr)
        else:
            print('person={} cstruct={}'.format(person, cstruct))


if __name__ == '__main__':
    sys.exit(main())