Protecting Dict and List Fields between class instances

Generally python allows sharing of class attributes between instances of that class. But I don’t think this this is the right behaviour for Umongo.

In this example the behaviour is expected, both instances of RegularClass share the sharable_field property.

class RegularClass():
    primitive_field = None
    sharable_field = []
    @classmethod
    def create(cls, update):
        new_object = cls()
        new_object.sharable_field.append(update)
        new_object.primitive_field = update
        return new_object

a = RegularClass.create(update=1)
b = RegularClass.create(update=2)

print(a.primitive_field, a.sharable_field)
# 1 [1, 2]
print(b.primitive_field, b.sharable_field)
# 2 [1, 2]

But even though Umongo documents give the same output (when list fields have a default value), it’s not what I expect.

import motor
from umongo import Instance
from umongo.document import Document
from umongo.fields import ListField, IntField

db = motor.motor_asyncio.AsyncIOMotorClient()
instance = Instance(db.data)

@instance.register
class UmongoClass(Document):
    primitive_field = None
    sharable_field = ListField(IntField(), default=[])
    @classmethod
    def create(cls, update):
        new_object = cls()
        new_object.sharable_field.append(update)
        new_object.primitive_field = update
        return new_object

a = UmongoClass.create(update=1)
b = UmongoClass.create(update=2)

print(a.primitive_field, a.sharable_field)
# 1 [1, 2]
print(b.primitive_field, b.sharable_field)
# 2 [1, 2]

It’s not what I expect because when I use SQLAlchemy or MongoEngine they do something different.

import datetime
from mongoengine import *
connect('mydb')

class BlogPost(Document):
    sharable_field = ListField(IntField(max_length=50))
    
    @classmethod
    def create(cls, update):
        new_object = cls()
        new_object.sharable_field.append(update)
        return new_object

a = BlogPost.create(update=1)
b = BlogPost.create(update=2)


print(a.sharable_field)
# [1]
print(b.sharable_field)
# [2]

We’ve patched this for ourselves by automatically reinitializing dict and list fields. But this seems like it should be changed upstream. What do you think, could the constructor for DocumentTemplates initialize a different object for each instance? Here is our patch in the class that all of our documents implement.

@classmethod
def create(cls,**kwargs):
    new_object = cls()
    for key, value in cls.schema.fields.items():
        if type(value) == ListField:
            new_object[key] = []
        if type(value) == DictField:
            new_object[key] = {}

[molten-firescar96]

I uploaded the wrong version, here is the equivalent code in MongoEngine. If it would be helpful I’m happy to make a minimum reproducible build for SQLAlchemy (or another library) as well.

import datetime
from mongoengine import *
connect('mydb')

class BlogPost(Document):
    sharable_field = ListField(IntField(max_length=50), default=[])
    
    @classmethod
    def create(cls, update):
        new_object = cls()
        new_object.sharable_field.append(update)
        return new_object

a = BlogPost.create(update=1)
b = BlogPost.create(update=2)

print(a.sharable_field)
# [1]
print(b.sharable_field)
# [2]

Okay, for our own development we need to protect against interns making this mistake so we will continue using either the patch I referenced above or something more explicit. I think we’ll be okay if umongo/marshmallow doesn’t change.

[lafrech]

Thanks.

If it would be helpful I’m happy to make a minimum reproducible build for SQLAlchemy (or another library) as well.

No, don’t bother.

Whatever patch you come up with, please share it here in case someone wants to do the same.