Recursive function to create hierarchical JSON object?

Question

I'm just not a good enough computer scientist to figure this out by myself :(

I have an API that returns JSON responses that look like this:

// call to /api/get/200
{ id : 200, name : 'France', childNode: [ id: 400, id: 500] } 
// call to /api/get/400
{ id : 400, name : 'Paris', childNode: [ id: 882, id: 417] } 
// call to /api/get/500
{ id : 500, name : 'Lyon', childNode: [ id: 998, id: 104] } 
// etc

I would like to parse it recursively and build a hierarchical JSON object that looks something like this:

{ id: 200,
  name: 'France', 
  children: [
     { id: 400,
       name: 'Paris',
       children: [...]
     },
     { id: 500,
       name: 'Lyon', 
       children: [...]
     } 
  ],
} 

So far, I have this, which does parse every node of the tree, but doesn't save it into a JSON object. How can I expand this to save it into the JSON object?

hierarchy = {}
def get_child_nodes(node_id):   
    request = urllib2.Request(ROOT_URL + node_id)
    response = json.loads(urllib2.urlopen(request).read())
    for childnode in response['childNode']:
        temp_obj = {}
        temp_obj['id'] = childnode['id']
        temp_obj['name'] = childnode['name']
        children = get_child_nodes(temp_obj['id'])
     // How to save temp_obj into the hierarchy?
get_child_nodes(ROOT_NODE)

This isn't homework, but maybe I need to do some homework to get better at solving this kind of problem :( Thank you for any help.

Answer 1

def get_node(node_id):   
    request = urllib2.Request(ROOT_URL + node_id)
    response = json.loads(urllib2.urlopen(request).read())
    temp_obj = {}
    temp_obj['id'] = response['id']
    temp_obj['name'] = response['name']
    temp_obj['children'] = [get_node(child['id']) for child in response['childNode']]
    return temp_obj

hierarchy = get_node(ROOT_NODE)

Answer 2

You could use this (a more compact and readable version)

def get_child_nodes(node_id):   
    request = urllib2.Request(ROOT_URL + node_id)
    response = json.loads(urllib2.urlopen(request).read())
    return {
       "id":response['id'],
       "name":response['name'],
       "children":map(lambda childId: get_child_nodes(childId), response['childNode'])
    }

get_child_nodes(ROOT_NODE)

Answer 3

You're not returning anything from each call to the recursive function. So, it seems like you just want to append each temp_obj dictionary into a list on each iteration of the loop, and return it after the end of the loop. Something like:

def get_child_nodes(node_id):   
    request = urllib2.Request(ROOT_URL + node_id)
    response = json.loads(urllib2.urlopen(request).read())
    nodes = []
    for childnode in response['childNode']:
        temp_obj = {}
        temp_obj['id'] = childnode['id']
        temp_obj['name'] = childnode['name']
        temp_obj['children'] = get_child_nodes(temp_obj['id'])
        nodes.append(temp_obj)
    return nodes

my_json_obj = json.dumps(get_child_nodes(ROOT_ID))

(BTW, please beware of mixing tabs and spaces as Python isn't very forgiving of that. Best to stick to just spaces.)

Answer 4

I had the same problem this afternoon, and ended up rejigging some code I found online.

I've uploaded the code to Github (https://github.com/abmohan/objectjson) as well as PyPi (https://pypi.python.org/pypi/objectjson/0.1) under the package name 'objectjson'. Here it is below, as well:

Code (objectjson.py)

import json

class ObjectJSON:

  def __init__(self, json_data):

    self.json_data = ""

    if isinstance(json_data, str):
      json_data = json.loads(json_data)
      self.json_data = json_data

    elif isinstance(json_data, dict):
      self.json_data = json_data

  def __getattr__(self, key):
    if key in self.json_data:
      if isinstance(self.json_data[key], (list, dict)):
        return ObjectJSON(self.json_data[key])
      else:
        return self.json_data[key]
    else:
      raise Exception('There is no json_data[\'{key}\'].'.format(key=key))

  def __repr__(self):
    out = self.__dict__
    return '%r' % (out['json_data'])

Sample Usage

from objectjson import ObjectJSON

json_str = '{ "test": {"a":1,"b": {"c":3} } }'

json_obj = ObjectJSON(json_str)

print(json_obj)           # {'test': {'b': {'c': 3}, 'a': 1}}
print(json_obj.test)      # {'b': {'c': 3}, 'a': 1}
print(json_obj.test.a)    # 1
print(json_obj.test.b.c)  # 3