How to serialise both positional and non-positional arguments of Python objects?

Question

How can I serialise test below:

class Foo:
    a = 0
    b = {}

    def __init__(self, a, b=None):
        self.a = a

        if b:
            self.b = b


test = Foo(1)
test.b['c'] = 2

so that the output is:

{"a": 1, "b": {"c": 2}}

I've tried:

print(json.dumps(test, default=lambda x: x.__dict__))

but it returns:

{"a": 1}

I understand that test.b['c'] = 2 does not add b or c to Foo.__dict__ , which is probably why x.__dict__ in the lambda doesn't pick them up. So is the answer one of:

1. Do not assign key-value pairs to arbitrary objects; use setattr instead.
2. Do not define arbitrary classes, if its property set can evolve at runtime; use a simple dict instead.

Answer 1

The problem here is test.b is not a instance variable. So when you serialize the object test using json.dumps , its not finding an instance variable b at all.

If you redefine the constructor like below:

class Foo:
    a = 0 #this is not instance variable - this is a class variable
    b = {} #this is not instance variable - this is a class variable

    def __init__(self, a, b=None):
        self.a = a
        self.b = {} #declared the instance variable b also
        if b:
            self.b = b


test = Foo(1)
test.b['c'] = 2

Now, if you run you get the desired output.