class Address(Serializer):
    id = IntegerField(required=True, identity=True)
    street = StringField(required=True)
    streetNumber = StringField(required=True)
    city = StringField(required=False)
    country = StringField(required=False)

class User(Serializer):
    _type = TypeField('user')
    id = IntegerField(required=True, identity=True)
    name = StringField(required=True)
    email = EmailField(required=True)
    tel = StringField(required=False, min_length=10, max_length=50)
    address = SerializerField(Address, required=True)

user = User(DATA)
user.dump()

{
  "_type": "user",
  "id": 1,
  "name": "Joe",
  "email": "joe@example.com",
  "tel": "+49 555 555 12",
  "address": {
    "id": 1,
    "street": "Street",
    "streetNumber": "5a",
    "city": "Berlin",
    "country": "Germany"
  }
}