Merge nested data with the same 'data_key'

from marshmallow import Schema, fields, EXCLUDE
from pprint import pprint

data = {
    'username' : "John",
    'address': {
        'phone': '88000111000',
        'street': 'Kolobko Str',
        'home': 15,
    },
}

class Address:
    def __init__(self, *args, **kwargs):
        self.street = kwargs.get('street')
        self.home = kwargs.get('home')

class Contacts:
    def __init__(self, *args, **kwargs):
        self.phone = kwargs.get('phone')

class User:
    def __init__(self, *args, **kwargs):
        self.name = kwargs.get('name')
        self.address = kwargs.get('address')
        self.contacts = kwargs.get('contacts')


class AddressSchema(Schema):
    street = fields.String()
    home = fields.Integer()

class ContactsSchema(Schema):
    phone = fields.String()

class UserSchema(Schema):

    name = fields.String(data_key='username')
    address = fields.Nested(AddressSchema, only=['home', 'street'], unknown=EXCLUDE)
    contacts = fields.Nested(ContactsSchema, only=['phone'], data_key='address', unknown=EXCLUDE)

schema = UserSchema()
user = schema.load(data)    
res = schema.dump(user)
pprint(res, indent=2)

code above show different results:

➜ python test_marshmallow/test_nested_fields.py
{'address': {'home': 15, 'street': 'Kolobko Str'}, 'username': 'John'}

➜ python test_marshmallow/test_nested_fields.py
{'address': {'phone': '88000111000'}, 'username': 'John'}

while expected:

{
    'username': 'John',
    'address': {
        'home': 15, 
        'street': 'Kolobko Str',
        'phone': '88000111000'
    }
}

marshmallow==3.0.0b16

[edelooff]

Having this raise an exception seems like it’s the correct solution, given that this is essentially a configuration error. Ideally this would be raised during creation of the schema, helping with visibility.

Quietly (and non-deterministically) doing stuff to the data going through the schema is likely to result in serious and hard to understand bugs.

[lafrech]

I just sent a PR to raise a ValueError in case of duplicate: https://github.com/marshmallow-code/marshmallow/pull/992.

@ehles, @edelooff, please have a look and comment if you have the time.