Bson ObjectId generating same value in python dataclass objects

Question

I am trying to generate an ObjectId that is unique for objects I make from a dataclass. Hovewer for every object I make from my class, it generates the same Id.

from dataclasses import dataclass
from bson import ObjectId

@dataclass
class B:
    id: ObjectId=ObjectId()

b =B()
b.id
>>ObjectId('600c9d09c889e41a182988b0')

c =B()
c.id
>>ObjectId('600c9d09c889e41a182988b0')

I do not understand this behaviour, is it due to the dataclass keeping the same default objectId reference everytime it inits the class?

Do you have a workaround?

Answer 1

Since the above dataclass would be initiated as:

class B:
    def __init__(self, x: ObjectId = ObjectId()):
        self.id=x

The default value for the class would always be the static ObjectId generated when the class is loaded. In the end, I suppose dynamic values should not be passed as init params.

The correct way would be to use __post_init__:

from dataclasses import field

@dataclass
class B:
    id: ObjectId = field(init=False)

    def __post_init__(self):
        self.id = ObjectId()

b =B()
b.id
>>ObjectId('60104262ee527a385cb44a11')

c =B()
c.id
>>ObjectId('6010426bee527a385cb44a12')