Grouping an array of objects by key in python

Question

Suppose I have an array of objects.

arr = [
        {'grade': 'A', 'name': 'James'},
        {'grade': 'B', 'name': 'Tom'},
        {'grade': 'A', 'name': 'Zelda'}
      ]

I want this result

{
   'A': [
            {'grade': 'A', 'name': 'James'},
            {'grade': 'A', 'name': 'Zelda'}
        ],
   'B': [ {'grade': 'B', 'name': 'Tom'} ]
}

Answer 1

Use a dict and setdefault:

setdefault(key[, default])

If key is in the dictionary, return its value. If not, insert key with a value of default and return default. default defaults to None.

arr2 = {}
for d in arr:
    t = arr2.setdefault(d['grade'], [])
    t.append(d)

>>> arr2
{'A': [{'grade': 'A', 'name': 'James'}, {'grade': 'A', 'name': 'Zelda'}],
 'B': [{'grade': 'B', 'name': 'Tom'}]}