Dynamically creating nested json objects with the same key name?

Question

I am working with a system that unfortunately; uses nested json objects with the same key name. Based on some input data based on 0 to n elements I need to create this object dynamically but not sure how to do this with python correctly.

Here is what my json looks like in this example there are 3 expressions..

{
    "metricExpressionTemplate": {
        "metricExpressionType": "Boolean",
        "operator": {
            "type": "PLUS"
        },
        "expression1": {
            "metricExpressionType": "Boolean",
            "operator": {
                "type": "PLUS"
            },
            "expression1": {
                "metricExpressionType": "Boolean",
                "operator": {
                    "type": "PLUS"
                },
                "expression1": {
                    "metricExpressionType": "Boolean",
                    "operator": {
                        "type": "PLUS"
                    }

                }
            }
        }
    }
}

I've been trying to dynamically create this but even when I try to add it manually..

import json

expression_object = {}
template_object = {
    "metricExpressionType": "Boolean",
    "operator": {
            "type": "PLUS"
        },
    }

expression_object["metricExpressionTemplate"]["expression1"] = template_object
expression_object["metricExpressionTemplate"]["expression1"]["expression1"] = template_object

print(json.dumps(expression_object))

I get an error saying "ValueError: Circular reference detected".

Now even if this did work imagine having to do this for 250 objects! In some type of for loop..

for i in range(0,200):
    # Add Expression objects dynamically..
    expression_object["metricExpressionType"]["expression1"] = template_object

There's no "get_json_root" or "append" method on a dict object that I am aware of. How the heck would I do this?

see sharper · Accepted Answer · 2018-03-26 03:25:51Z

Instead of assigning template_object to ["metricExpressionTemplate"]["expression1"] and ["metricExpressionTemplate"]["expression1"]["expression1"], use a factory method to generate new template objects each time. So:

import json

def newTemplateObject():
    t = {
        "metricExpressionType": "Boolean",
        "operator": {
                "type": "PLUS"
            },
        }
    return t


expression_object = {}
expression_object["metricExpressionTemplate"] = newTemplateObject()
expression_object["metricExpressionTemplate"]["expression1"] = newTemplateObject()
expression_object["metricExpressionTemplate"]["expression1"]["expression1"] = newTemplateObject()
print(json.dumps(expression_object))

As for adding the template_object when you have an object of unknown depth, you could use a recursive method, with each call checking whether the object passed to it has a key "expression1":

def addExpression(obj):
    if "expression1" in obj.keys():
        addExpression(obj["expression1"])
    else:
        obj["expression1"] = newTemplateObject()

Synook · Accepted Answer · 2018-03-26 03:25:29Z

The error is occurring because you are attempting to reuse the same object inside itself. This can be overcome by generating a new dictionary each time it is needed. Then this can be performed in a loop through a recursive system, for example:

import json

def get_contents():
  return {
    "metricExpressionType": "Boolean",
      "operator": {
          "type": "PLUS"
      }
  }

def add_contents(d, contents, remaining):
  d['expression1'] = contents()
  if remaining > 0:
    add_contents(d['expression1'], contents, remaining - 1)

objects = []
templates = ['metricExpressionTemplate', 'b', 'c']

for template_name in templates:
  d = {template_name: get_contents()}
  add_contents(d, get_contents, 2)
  objects.append(d)

print json.dumps(objects, indent=2)

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