Dealing with scope in a recursive Python function

Question

I have a JSON input file that looks like this:

{"nodes": [
    {"properties": {
        "id": "rootNode",
        "name": "Bertina Dunmore"},
      "nodes": [
        {"properties": {
            "id": 1,
            "name": "Gwenneth Rylett",
            "parent_id": "rootNode"},
          "nodes": [
            {"properties": {
                "id": 11,
                "name": "Joell Waye",
                "parent_id": 1}},
            {"properties": {
                "id": 12,
                "name": "Stan Willcox",
                "parent_id": 1}}]},
        {"properties": {
            "id": 2,
            "name": "Delbert Dukesbury",
            "parent_id": "rootNode"},
          "nodes": [
            {"properties": {
                "id": 21,
                "name": "Cecil McKeever",
                "parent_id": 2}},
            {"properties": {
                "id": 22,
                "name": "Joy Obee",
                "parent_id": 2}}]}]}]}

I want to get the nested properties dictionaries into a (flat) list of dictionaries. Creating a recursive function that will read this dictionaries is easy:

def get_node(nodes):
    for node in nodes:
        print(node['properties'])
        if 'nodes' in node.keys():
            get_node(node['nodes'])

Now, I’m struggling to append these to a single list:

def get_node(nodes):
    prop_list = []
    for node in nodes:
        print(node['properties'])
        prop_list.append(node['properties'])
        if 'nodes' in node.keys():
            get_node(node['nodes'])
    return prop_list

This returns [{'id': 'rootNode', 'name': 'Bertina Dunmore'}], even though all properties dictionaries are printed. I suspect that this is because I’m not handling the function scope properly.

Can someone please help me get my head around this?

Asked By: mrgou

||

Source

Answer 1

You need to combine the prop_list returned by the recursive call with the prop_list in the current scope. For example,

def get_node(nodes):
    prop_list = []
    for node in nodes:
        print(node['properties'])
        prop_list.append(node['properties'])
        if 'nodes' in node.keys():
            prop_list.extend(get_node(node['nodes']))
    return prop_list

Answered By: chepner

Answer 2

your problem is that every time you call get_node, the list where you append is initialized again. you can avoid this by passing the list to append in the recursive function

Moreover, I think would be nice to use dataclass to deal with this problem,

from dataclasses import dataclass
from typing import Union


@dataclass
class Property:
    id: int
    name: str
    parent_id: Union[str, None] = None
    
        
def explore_json(data, properties: list=None):
    if properties is None:
        properties = []
    for key, val in data.items():
        if key == "nodes":
            for node in val:
                explore_json(node, properties)
        elif key == "properties":
            properties.append(Property(**val))
    return properties

explore_json(data)

output

[Property(id='rootNode', name='Bertina Dunmore', parent_id=None),
 Property(id=1, name='Gwenneth Rylett', parent_id='rootNode'),
 Property(id=11, name='Joell Waye', parent_id=1),
 Property(id=12, name='Stan Willcox', parent_id=1),
 Property(id=2, name='Delbert Dukesbury', parent_id='rootNode'),
 Property(id=21, name='Cecil McKeever', parent_id=2),
 Property(id=22, name='Joy Obee', parent_id=2)]

Answered By: Lucas M. Uriarte

Answer 3

With that:

def get_node(prop_list, nodes):
    for node in nodes:
        print(node['properties'])
        prop_list.append(node['properties'])
        if 'nodes' in node.keys():
            get_node(prop_list, node['nodes'])

You can just do:

prop_list = []
get_node(prop_list, <yourdictnodes>)

Should alter prop_list into:

{'id': 'rootNode', 'name': 'Bertina Dunmore'}
{'id': 1, 'name': 'Gwenneth Rylett', 'parent_id': 'rootNode'}
{'id': 11, 'name': 'Joell Waye', 'parent_id': 1}
{'id': 12, 'name': 'Stan Willcox', 'parent_id': 1}
{'id': 2, 'name': 'Delbert Dukesbury', 'parent_id': 'rootNode'}
{'id': 21, 'name': 'Cecil McKeever', 'parent_id': 2}
{'id': 22, 'name': 'Joy Obee', 'parent_id': 2}

Answered By: Limtorak

Dealing with scope in a recursive Python function

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