How to exclude specific fields on serialization with jsonpickle?

Question

I'm using SQLAlchemy extension with Flask. While serializing my models (which are also used for database operations) using jsonpickle, I want some specific attributes to be ignored. Is there a way that allows me to set those rules?

SQLAlchemy adds an attribute named _sa_instance_state to the object. In a word, I do not want this field to be in the JSON output.

Answer 1

You cannot tell the default class pickler to ignore something, no.

jsonpickle does support the pickle module __getstate__ and __setstate__ methods. If your classes implement those two methods, whatever is returned is then used by jsonpickle to represent the state instead. Both methods do need to be implemented.

If __getstate__ is not implemented, jsonpickle uses the __dict__ attribute instead, so your own version merely needs to use that same dictionary, remove the _sa_instance_state key and you are done:

def __getstate__(self):
    state = self.__dict__.copy()
    del state['_sa_instance_state']
    return state

def __setstate__(self, state):
    self.__dict__.update(state)

Whatever __getstate__ returns will be processed further, recursively, there is no need to worry about handling subobjects there.

If adding __getstate__ and __setstate__ is not an option, you can also register a custom serialization handler for your class; the disadvantage is that while __getstate__ can get away with just returning a dictionary, a custom handler will need to return a fully flattened value.