How to group a list of objects into a dictionary of lists based on id

May 18, 2022

I have a list of items with id. I want to group the items into separate lists according to a shared id.

For example, this is my original list:

[
  {
    "id": "x",
    "v": "a"
  },
  {
    "id": "x",
    "v": "b"
  },
  {
    "id": "y",
    "v": "c"
  }
]

I would like to output:

{
  "x": [
    {
      "id": "x",
      "v": "a"
    },
    {
      "id": "x",
      "v": "b"
    }
  ],
  "y": [
    {
      "id": "y",
      "v": "c"
    }
  ]
}

This is how I would do this with javascript:

const output = input.reduce((obj, item) => {
    if (!obj[item.id]) obj[item.id] = [];
    obj[item.id].push(item);
    return obj;
}, {})

I can’t find how to do this in python in a short and elegant way like javascript…

>Solution :

Use dict.setdefault:

>>> result = {}
>>> for dct in your_list:
...     result.setdefault(dct['id'], []).append(dct)
...
>>> result
{'x': [{'id': 'x', 'v': 'a'}, {'id': 'x', 'v': 'b'}], 'y': [{'id': 'y', 'v': 'c'}]}

Or collections.defaultdict:

>>> from collections import defaultdict
>>> result = defaultdict(list)
>>> for dct in your_list:
...     result[dct['id']].append(dct)
...
>>> result
defaultdict(<class 'list'>, {'x': [{'id': 'x', 'v': 'a'}, {'id': 'x', 'v': 'b'}], 'y': [{'id': 'y', 'v': 'c'}]})