How to Group data by a key in a dictionary in Python

Problem –

You have a list of dictionaries and you want to group the data by a key in the dictionary.

customers = [{'customer':1, 'net sales': 39.5, 'method':'Discover'},
            {'customer':2, 'net sales': 102.4, 'method':'Proprietary'},
            {'customer':3, 'net sales': 22.5, 'method': 'Proprietary'},
            {'customer':4, 'net sales': 100.4, 'method': 'Proprietary'},
            {'customer':5, 'net sales': 54.0, 'method': 'MasterCard'},
            {'customer':6, 'net sales': 75.0, 'method': 'Discover'}]

Solution –

Let’s say that you want to group the data by the method of payment. This can be done using the itertools.groupby function. First we have to sort the data by the desired key in this case ‘method’ and then use itertools.groupby.

In [10]: from operator import itemgetter

In [11]: from itertools import groupby

In [12]: # sort the data by desired key first

In [13]: customers.sort(key=itemgetter('method'))

In [14]: # then iterate in groups

In [15]: for method, items in groupby(customers, key=itemgetter('method')):
    ...:     print(method)
    ...:     for i in items:
    ...:         print('    ', i)
    ...:         
Discover
     {'customer': 1, 'net sales': 39.5, 'method': 'Discover'}
     {'customer': 6, 'net sales': 75.0, 'method': 'Discover'}
MasterCard
     {'customer': 5, 'net sales': 54.0, 'method': 'MasterCard'}
Proprietary
     {'customer': 2, 'net sales': 102.4, 'method': 'Proprietary'}
     {'customer': 3, 'net sales': 22.5, 'method': 'Proprietary'}
     {'customer': 4, 'net sales': 100.4, 'method': 'Proprietary'}