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'} ]
}

Asked By: hendrixchord

||

Source

Answer 1

Using dict.setdefault we can do this:

import json
gradeList = [
    {"grade": 'A', "name": 'James'},
    {"grade": 'B', "name": 'Tom'},
    {"grade": 'A', "name": 'Zelda'}
]
gradeDict = {}
for d in gradeList:
    gradeDict.setdefault(d["grade"], []).append(d)

print(json.dumps(gradeDict, indent=4))

Output:

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

Answered By: Hampus Larsson

Answer 2

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'}]}

Answered By: Corralien

Answer 3

I would use a pd.Dataframe and do it like this:

import pandas as pd
df = pd.Dataframe(arr)    
for index, group in df.groupby('grade'):
    print(group)

Instead of print(group) you can write the data to whatever you need it, I suppose it is not necessarily a dict like you described.

Answered By: qkfsbxjayiedbe

Answer 4

I would do a simple loop like this:

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

grouped_grades = {}

for item in arr:
    if item['grade'] not in grouped_grades:
        grouped_grades[item['grade']] = []
        
    grouped_grades[item['grade']].append(item)

print(grouped_grades)

Output:

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

Answered By: sjjk001

Answer 5

I think that the easiest way is to use defaultdict. Then you could convert the result back into an ordinary dict if you need to by simply passing it in the constructor like dict(output).

from collections import defaultdict
output = defaultdict(lambda: [])

for item in arr:
    output[item['grade']].append(item)

Answered By: emarcus

Answer 6

You can use itertools.groupby

>>> keyfunc = lambda item: item['grade']
>>> {k:list(v) for k,v in itertools.groupby( sorted(arr,key=keyfunc) , keyfunc) }
{'A': [{'grade': 'A', 'name': 'James'}, {'grade': 'A', 'name': 'Zelda'}], 'B': [{'grade': 'B', 'name': 'Tom'}]}

Answered By: napuzba

Grouping an array of objects by key in python

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