Python aggregate dictionary by key

I'm quite new to Python and just started working with dictionaries. I have the following question:

Given the following dictionary:

{'Berlin': {'Country': 'Germany', 'Population': 3.502},
'New York': {'Country': 'USA', 'Population': 8.406},
'Munich': {'Country': 'Germany', 'Population': 1.388}, ... }

How could I sum the population over countries? Thank you!

Avihoo Mamka

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asked Dec 13, 2016 at 10:06

Patrick BaladaPatrick Balada

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You can use itertools.groupby() to group your data by country like below:

from itertools import groupby

my_dict = {'Berlin': {'Country': 'Germany', 'Population': 3.502}, 'New York': {'Country': 'USA', 'Population': 8.406}, 'Munich': {'Country': 'Germany', 'Population': 1.388}}

population = {}

for k,v in groupby(sorted(my_dict.items()), key=lambda i:i[1]['Country']):
    population[k] = sum(item[1]['Population'] for item in list(v))

Output:

>>> population
{'Germany': 4.89, 'USA': 8.406}

answered Dec 13, 2016 at 10:25

country_to_population = {}
for city, data in dict.iteritems():
    country = data['Country']
    if country not in country_to_population:
        country_to_population[country] = 0
    country_to_population[country] += data['Population']

You iterate the main dictionary and add population to a new dictionary that maps a country to population.

If using Python 3, do items() instead of iteritems().

answered Dec 13, 2016 at 10:09

Rok PovsicRok Povsic

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1

You can go inside the dict iterating on the key:

dict = {'Berlin': {'Country': 'Germany', 'Population': 3502},
'New York': {'Country': 'USA', 'Population': 8406},
'Munich': {'Country': 'Germany', 'Population': 1388}}
country = []
sum = {}
for key in dict:
    coun = dict[key]['Country']
    if coun in country:
        pass
    else:
        country.append(dict[key]['Country'])
        sum[coun] = 0

    sum[coun] += dict[key]['Population']

for key in sum:
    print(key, sum[key])    

answered Dec 13, 2016 at 10:10

Marco smdmMarco smdm

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You can use defaultdict from collections module, with int assigned to it in order to have a default value of 0 to each country you are about to sum.

The code looks like:

from collections import defaultdict

country_dict = {"Berlin":{"Country":"Germany","Population":3.502},"Munich":{"Country":"Germany","Population":1.388},"New York":{"Country":"USA","Population":8.406}}

sum_countries_dict = defaultdict(int)
for item in country_dict.values():
    sum_countries_dict[item['Country']] += item['Population']

And the result is:

>>> country_dict
{'Berlin': {'Country': 'Germany', 'Population': 3.502}, 'New York': {'Country': 'USA', 'Population': 8.406}, 'Munich': {'Country': 'Germany', 'Population': 1.388}}
>>> sum_countries_dict
defaultdict(, {'Germany': 4.89, 'USA': 8.406})
>>> sum_countries_dict['Germany']
4.89
>>> sum_countries_dict['USA']
8.406

answered Dec 13, 2016 at 10:13

Avihoo MamkaAvihoo Mamka

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How to group dictionary in Python by key?

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