根据字典列表中的两个值获取字典列表

Question

The list of dictionaries looks like:

lst_dict = [{'type': 'email', 'count': '180', 'date': '2019-04-22'},
            {'type': 'text', 'count': '12', 'date': '2018-12-20'},
            {'type': 'email', 'count': '150','date': '2018-12-20'}]

I am interested in getting the unique list of elements by type and with latest date. So, expected outcome is:

[{'type': 'email', 'count': '180', 'date': '2019-04-22'},
{'type': 'text', 'count': '12', 'date': '2018-12-20'}]

To get the unique elements by type I did:

list({item['type']: item for item in lst_dict}.values())

The outcome is:

[{'type': 'email', 'count': '150', 'date': '2018-12-20'},
 {'type': 'text', 'count': '12', 'date': '2018-12-20'}]

But it does not give the element by latest date.Tried different possibilities but got no luck.

Answer 1

Here's one way using itertools.groupby :

from itertools import groupby
l = sorted(lst_dict, key=lambda x: x['type'])
[max(list(v), key=lambda x: x['date']) for _,v in groupby(l, key=lambda x: x['type'])]

[{'count': '180', 'date': '2019-04-22', 'type': 'email'},
 {'count': '12', 'date': '2018-12-20', 'type': 'text'}]

Answer 2

If you want to keep your code you can sort the list by date in advance, it seems like your line of code will take the last entry in the list that matches the same type.

lst_dict = [{'type': 'email', 'count': '180', 'date': '2019-04-22'},
            {'type': 'text', 'count': '12', 'date': '2018-12-20'},
            {'type': 'email', 'count': '150','date': '2018-12-20'}]
lst_dict.sort(key=lambda x: x['date'])
print(list({item['type']: item for item in lst_dict}.values()))

Answer 3

My solution:

d = {}
for elem in lst_dict:
    if elem['type'] not in d: 
        d[elem['type']] = elem
    if elem['date'] > d[elem['type']]['date']: 
        d[elem['type']]=elem

You create a new dict, add a new entry for each type, and replace that entry if you have a newer date (only works in that specific date format, not really recommended).