In Python, is there a way to use json.dumps on an object that uses __slots__?

Question

When I try to use json.dumps on an object of a class that uses __slots__ , I get "...is not JSON serializable," or possibly an AttributeError that __dict__ is missing. How can I get this to work? It seems that __slots__ should tell the interpreter to use a virtual dictionary for compatibility.

import json

class Foo:
     __slots__ = ["bar"]
     def __init__(self):
         self.bar = 0

json.dumps(Foo())

Answer 1

Plain, vanilla json.dumps() doesn't support custom classes, period. It doesn't matter if they use __slots__ or not here.

A popular way to handle custom classes is to use a hook that returns their __dict__ attribute instead, and that obviously won't work here. You'd have to find another way to serialise such objects.

One way would be for such objects to have a dedicated method:

class Foo:
     __slots__ = ["bar"]
     def __init__(self):
         self.bar = 0

     def json_serialize(self):
         return {'bar': self.bar}

and use that in your default hook:

json.dumps(Foo(), default=lambda o: o.json_serialize())