[Python] How to make a field read-only in a `mongoengine` Document while allowing conditional...

I have a mongoengine document such as:

from mongoengine.document import Document
from mongoengine import StringField

class Class_A(Document):
field_a = StringField()
field_b = StringField()


I'd like to lock field_b so it cannot be altered, e.g.

var = <fetch Class_a document from DB>
var.field = 'abc'


would raise an error.

This on itself is not a problem, but I'd like to be able to set field_b when field_a is set. To give an example, field_a could be some data and field_b would be computed hash for this data - user can set the data, but not the hash for it (it should be only set automatically when data is assigned).

I tried using __setattr__/__dict__, but mongoengine seems to be doing some attributes magic behind the scene and I couldn't make it work. I also had an idea to subclass StringField and use a metaclass to wrap it's __setattr__, with similar effect.

How to achieve such a behaviour?

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