How to add a key and value using list and dict comprehension in list of dictionaries?

Question

I have a list of dictionaries which is,

ip_list = [{'1403': [-56, -58], 'data': '1'},
           {'1403': [-56, -58], 'data': '0'}]

Now I need to add a new key(i.e., "mac") and value(i.e., "xyz") in a dictionary if dictionary contains 'data' = 1 and the outcome should be,

expected_outcome = [{'1403': [-56, -58], 'data': '1', 'mac':'xyz'},
                    {'1403': [-56, -58], 'data': '0'}]

I have tried with the,

list_dict_comp = [dict(item, **{'mac':'xyz'}) for item in ip_list]

Whereas, the above expression gives,

list_dict_comp = [{'1403': [-56, -58], 'data': '1', 'mac':'xyz'},
                  {'1403': [-56, -58], 'data': '0', 'mac':'xyz'}]

Can anyone help me out to achieve the "expected_outcome" using both list and dict comprehension together in a single expression?

This is not a problem you should solve with a list or dict comprehension. Just write a regular ol' loop. — Aran-Fey, Apr 11 '19 at 07:18

Devesh Kumar Singh · Accepted Answer · 2019-04-11T08:35:27.443

The reason you get

list_dict_comp = [{'1403': [-56, -58], 'data': '1', 'mac':'xyz'},
                  {'1403': [-56, -58], 'data': '0', 'mac':'xyz'}]

is because you are adding {'mac':'xyz'} to every element in the list.

Why not make your life easier and just iterate through ip_list, and add {'mac':'xyz'} if data is present in the keys of an element of ip_list, and the value for data is '1'

ip_list = [{'1403': [-56, -58], 'data': '1'},
           {'1403': [-56, -58], 'data': '0'}]

for ip in ip_list:
    if ip.get('data') == '1':
        ip['mac'] = 'xyz'
print(ip_list)
#[{'1403': [-56, -58], 'data': '1', 'mac': 'xyz'}, {'1403': [-56, -58], 'data': '0'}]

score 1 · Answer 2 · answered Apr 11 '19 at 07:37

1

ip_list = [{'1403': [-56, -58], 'data': '1'},
           {'1403': [-56, -58], 'data': '0'}]
res = [dict(item, **{'mac':'xyz'}) if 'data' in item and item['data'] == '1' else item for item in ip_list]
print(res)
# [{'1403': [-56, -58], 'data': '1', 'mac': 'xyz'}, {'1403': [-56, -58], 'data': '0'}]

answered Apr 11 '19 at 07:37

Travis

109
5

1

Provide some explanation to the code you write, so that next time if someone reads the solution, he knows what is happening – Devesh Kumar Singh Apr 11 '19 at 07:49
@DeveshKumarSingh It's actually better without an explanation, because that makes it more obvious what an unreadable mess this really is. Hopefully people will avoid using this. – Aran-Fey Apr 11 '19 at 08:23
Agreed! A lot of times list comprehension is a pretty unreadable jumbled mess – Devesh Kumar Singh Apr 11 '19 at 08:37
`dict(item, **{'mac':'xyz'})` can be rewritten as `dict(item, mac='xyz')` – Georgy Apr 11 '19 at 08:59
Also `if 'data' in item and item['data'] == '1'` can be shortened to `if item.get('data') == '1'` – Georgy Apr 11 '19 at 09:08

cph_sto · Answer 3 · 2019-04-11T07:48:27.937

1

Use list comprehensions to solve this with the help of if-else. You can add elements to the dictionary using update() function -

ip_list = [{'1403': [-56, -58], 'data': '1'}, {'1403': [-56, -58], 'data': '0'}]

[i.update({'mac': 'xyz'}) if i['data']=='1' else i for i in ip_list]         

print(ip_list)
     [{'1403': [-56, -58], 'data': '1', 'mac': 'xyz'},
      {'1403': [-56, -58], 'data': '0'}]

edited Apr 11 '19 at 07:48

answered Apr 11 '19 at 07:38

cph_sto

7,189
12
42
78

Don't use list comprehensions with side effects: [Is it Pythonic to use list comprehensions for just side effects?](https://stackoverflow.com/questions/5753597/is-it-pythonic-to-use-list-comprehensions-for-just-side-effects) – Georgy Apr 11 '19 at 09:19
@Georgy Thanks for bringing it to my notice. – cph_sto Apr 11 '19 at 09:20

How to add a key and value using list and dict comprehension in list of dictionaries?

3 Answers3