Python marshmallow tree structure with polymorphism

问题

I have the following code for a tree structure:

class Node:
    def __init__(self, node_id: str):
        self.node_id = node_id
        self.children = []

    def add_child(self, node: 'Node'):
        if isinstance(node, Node):
            self.children.append(node)


class ValueNode(Node):
    def __init__(self, value: bool, **kwargs):
        Node.__init__(self, **kwargs)
        self.value = value


class LogicNode(Node):
    def __init__(self, logic_operator: str, **kwargs):
        Node.__init__(self, **kwargs)
        self.logic_operator = logic_operator


a = Node("a")
b = LogicNode("or", node_id="b")
c = ValueNode(True, node_id="c")
d = ValueNode(False, node_id="d")

b.add_child(c)
b.add_child(d)
a.add_child(b)

How can I serialize object a into json and back again into a python object while keeping the correct types and tree structure?

What I tried:

I am trying to serialize this tree structure into json in order to send to an ui through an API. I found about marshmallow but unfortunately, I am unable to figure out how to do it. I have this as schemas atm.

class NodeSchema(Schema):
    node_id = fields.String()
    children = fields.Nested('self', many=True)

    @post_load
    def load(self, data):
        return Node(**data)


class ValueNodeSchema(NodeSchema):
    value = fields.Boolean()


class LogicNodeSchema(NodeSchema):
    logic_operator = fields.String()

Here the problem is that when serialized into json only the attributes of Node are there even if the children are logical node or value node (kinda expected). I found about marshmallow-oneofschema here github but I am unable to use it because with it I have to replace:

# In NodeSchema
children = fields.Nested('self', many=True)

with:

class SomeNodeSchema(OneOfSchema):
    type_schemas = {
        'node': NodeSchema,
        'logic_node': LogicNodeSchema,
        'value_node': ValueNodeSchema,
    }

    def get_obj_type(self, obj):
        if isinstance(obj, LogicNode):
            return 'logic_node'
        if isinstance(obj, ValueNode):
            return 'value_node'
        elif isinstance(obj, Node):
            return 'node'

# In NodeSchema
children = fields.Nested(SomeNodeSchema, many=True)

but this leads to a circular import which makes it impossible.

回答1:

I found about marshmallow-oneofschema

Although not integrated to the core, I think marshmallow-oneofschema is the recommended way to do polymorphism.

this leads to a circular import

You may pass the name as string to solve the circular import issue.

    children = fields.Nested('SomeNodeSchema', many=True)

来源：https://stackoverflow.com/questions/55092906/python-marshmallow-tree-structure-with-polymorphism

标签

python

serialization

polymorphism

marshmallow