Sorting JSON object(s) into a Hierarchy

Question

I need to sort out a JSON array into a Hierarchy, here my JSON file, it\'s never ordered but follow structure:

{
  \"name\":\"Folder 2\",
  \"id\":\"zRDg\",
         


        

Answer 1

What you have here is a graph (probably just a tree) with connections between nodes represented by a parent key. This is known as an adjacency list.

You want to produce a tree structure with links to other nodes through a children key which is a list of other nodes.

To convert an adjacency list into a tree, you first need to have a way to retrieve a node by its id, so the first step is to create a dict of every node, keyed by id.

Then you have to go through the list again and add children to their parents.

Finally, make a list of nodes without parents (the root nodes).

Note that you don't need a recursive algorithm at all.

We can combine some of these steps to avoid going through the list multiple times. Code below:

def nest_list(L):
    """Modify list of associative dicts into a graph and return roots of graph

    Association is via a 'parent' key indicating a corresponding 'id'.
    Items in the list *will be modified*.
    Children will be placed in a list for a 'children' key
    Items without children will have no 'children' key
    'parent' keys will be removed.

    Returned list is the full list of items which are not children of
    any other items.
    """
    idx = {}
    children = []
    root = []
    # first pass: index items by id
    # if the item has no parents, put it in the 'root' list
    # otherwise add it to the 'children' list which we will
    # process in a second pass
    for item in L:
        idx[item['id']] = item
        if 'parent' in item:
            children.append(item)
        else:
            root.append(item)

    # second pass: find the parents of our children
    for child in children:
        parent = idx[child['parent']]
        del child['parent']
        try:
            # retrieve the list of children for this
            # parent
            childlist = parent['children']
        except KeyError:
            # this item has received no children so far
            # so we need to make a 'children' key and list
            # for it.
            parent['children'] = childlist = []
        # append this child to the parent's list of children
        childlist.append(child)

    #all done: return new root list
    return root

Code in action:

oldlist = [{
  "name":"Folder 2",
  "id":"zRDg",
  "parent":"OY00",
  "type":"folder"
},
{
  "name":"Folder 1",
  "id":"OY00",
  "type":"folder"
},
{
  "name":"Folder 3",
  "id":"ZDE1",
  "type":"folder"
},
{
  "name":"DX00025.jpg",
  "id":"9Xdd",
  "parent":"OY00",
  "type":"jpeg"
}]

expected = [{
  "name":"Folder 1",
  "id":"OY00",
  "type":"folder",
  "children": [{
    "name":"Folder 2",
    "id":"zRDg",
    "type":"folder"
    },
    {
    "name":"DX00025.jpg",
    "id":"9Xdd",
    "type":"jpeg"
  }]
},
{
    "name":"Folder 3",
    "id":"ZDE1",
    "type":"folder"
}]


from pprint import pprint
newlist = nest_list(oldlist)
pprint(newlist)
assert newlist == expected