Python Dictionary comprehension for a list of dictionaries

Question

I want to create a dictionary from the following list

[{'fips': '01001', 'state': 'AL', 'name': 'Autauga County'}, {'fips': '20005', 'state': 'KS', 'name': 'Atchison County'}, {'fips': '47145', 'state': 'TN', 'name': 'Roane County'}]

The result should have the name as the key and 'United States' as the value.

eg:

{'Autauga County': 'United States', 'Atchison County' : 'United States',  'Roane County' : 'United States'}

I can do this with a couple of for loops but i want to learn how to do it using Dictionary Comprehensions.

You might want to specify which Python version you are using — Alan Kavanagh, Jan 25 '17 at 11:18

Jamie Phan · Accepted Answer · 2017-02-24T06:08:24.547

11

in_list = [{'fips': '01001', 'state': 'AL', 'name': 'Autauga County'}, 
           {'fips': '20005', 'state': 'KS', 'name': 'Atchison County'},
           {'fips': '47145', 'state': 'TN', 'name': 'Roane County'}]

out_dict = {x['name']: 'United States' for x in in_list if 'name' in x}

Some notes for learning:

Comprehensions are only for Python 2.7 onwards
Dictionary comprehensions are very similar to list comprehensions except with curly braces {} (and keys)
In case you didn't know, you can also add more complicated control-flow after the for loop in a comprehension such as [x for x in some_list if (cond)]

For completeness, if you can't use comprehensions, try this

out_dict = {}

for dict_item in in_list:
    if not isinstance(dict_item, dict):
        continue

    if 'name' in dict_item:
        in_name = dict_item['name']
        out_dict[in_name] = 'United States'

As mentioned in the comments, for Python 2.6 you can replace the {k: v for k,v in iterator} with:

dict((k,v) for k,v in iterator)

You can read more about this in this question

Happy Coding!

edited Feb 24 '17 at 06:08

answered Jan 25 '17 at 11:16

Jamie Phan

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Oh my bad - so used to Python 3. I must say though, pretty new here and that's the fastest set of downvotes I've gotten - is it because of the lack of Python 2 support or is there something else? – Jamie Phan Jan 25 '17 at 11:20
@tehjoker yes, well, dictionary comprehensions only work for python 2.7+ so it's sort of a moot point... – juanpa.arrivillaga Jan 25 '17 at 11:23
Understood haha (it was really fast though), thought I did something terribly wrong. Will keep that in mind. Thanks! – Jamie Phan Jan 25 '17 at 11:24
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I'm completely puzzled why you got downvotes. Don't feel bad for writing a Python 2.7 or 3 answer, without a version number in the question Python 3 is a good assumption. – Georg Schölly Jan 25 '17 at 11:24
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I am puzzled too. I think it is safe to assume that when the question asks about dictionary comprehensions that we are talking about a Python version that supports dictionary comprehensions... – juanpa.arrivillaga Jan 25 '17 at 11:24
1

Haha thanks guys - I'm not particularly fussed with rep, I think answering questions is a great way to learn coding - and downvotes just make sure you produce the best code! On a side note, someone should give an answer with both Python 2.x and 3.x support, I look forward to it (haven't done Python 2 for a while) – Jamie Phan Jan 25 '17 at 11:28
Seems like it's a valid solution, I took the pitch fork out too quick once I seen the Error on my screen! I've changed my vote and began downloading Python 2.7+.....happy coding! – Alan Kavanagh Jan 25 '17 at 11:29
@K.J.Phan 2.6 support is quite simple, you can replace `{k: v for k,v in iterator}` with `dict((k, v) for k,v in iterator)`. Basically just passing a iterator over tuples to the dictionary constructor,, – Georg Schölly Jan 25 '17 at 21:40

score 2 · Answer 2 · answered Jan 25 '17 at 11:21

Here's a little solution working for both python2.7.x and python 3.x:

data = [
    {'fips': '01001', 'state': 'AL', 'name': 'Autauga County'},
    {'fips': '20005', 'state': 'KS', 'name': 'Atchison County'},
    {'fips': '47145', 'state': 'TN', 'name': 'Roane County'},
    {'fips': 'xxx', 'state': 'yyy'}
]

output = {item['name']: 'United States' for item in data if 'name' in item}
print(output)

score -1 · Answer 3 · edited May 23 '17 at 11:46

The loop/generator version is:

location_list = [{'fips': '01001', 'state': 'AL', 'name': 'Autauga County'},
        {'fips': '20005', 'state': 'KS', 'name': 'Atchison County'},
        {'fips': '47145', 'state': 'TN', 'name': 'Roane County'}]
location_dict = {location['name']:'United States' for location in location_list}

Output:

{'Autauga County': 'United States', 'Roane County': 'United States',
 'Atchison County': 'United States'}

If you search on Stackoverflow for dictionary comprehension, solutions using the { } generator expression start to show up: Python Dictionary Comprehension

`dict` and `list` are poor names to use as they shadow the built-ins. — Tagc, Jan 25 '17 at 11:21

score -2 · Answer 4 · edited Jan 25 '17 at 11:25

-2

That should do the trick for you

states_dict = [{'fips': '01001', 'state': 'AL', 'name': 'Autauga County'}, {'fips': '20005', 'state': 'KS', 'name': 'Atchison County'}, {'fips': '47145', 'state': 'TN', 'name': 'Roane County'}]

{states_dict[i]['name']:'United States' for i, elem in enumerate(states_dict)}

edited Jan 25 '17 at 11:25

Tagc

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answered Jan 25 '17 at 11:25

BlackHawk

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Just to get better understanding, any issues with that ?. I believe it works fine and I tested it – BlackHawk Jan 25 '17 at 11:27
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@BlackHawk It's a confusing and inelegant way to accomplish something that can be done much simpler (see top-rated answers). – Tagc Jan 25 '17 at 11:33
Thanks for this and sorry my dump answer. I'm still learning and was trying to help only. – BlackHawk Jan 25 '17 at 11:45

Python Dictionary comprehension for a list of dictionaries

4 Answers4