How to manage a python dictionary with three items and add the last one?

I have a dictionary in python like this:

x = {country:{city:population}:......}

and I want to create a new dictionary like y = {country:cities_population}, where cities_population adds all the population in every city in every country and I really don't know how to do it.

I tried this:

for country in x:
 for city, population in x[country].iteritems():
  if not country in y:
   y[country] = {}
  y[countr开发者_JAVA技巧y] += population

I check for a dictionary with only one key and one value but I don't understand how to manage a three items dictionary... help me please!!!! :)

Well, how about:

 y = {  }
 for country, cities in x.iteritems():
     y[country] = sum(cities.values())

I'm going to assume you want to sum up more than just the city populations for each country:

>>> attributes = ['population', 'gdp', 'murders']
>>> x = {'usa': {'chicago': dict(zip(attributes, [10, 100, 1000])), 'nyc':dict(zip(attributes, [20, 200, 2000]))}, 'china': {'shanghai': dict(zip(attributes, [9, 90, 900])), 'nagasaki': dict(zip(attributes, [2, 20, 200]))}}
>>> x
{'china': {'shanghai': {'gdp': 90, 'murders': 900, 'population': 9}, 'nagasaki': {'gdp': 20, 'murders': 200, 'population': 2}}, 'usa': {'nyc': {'gdp': 200, 'murders': 2000, 'population': 20}, 'chicago': {'gdp': 100, 'murders': 1000, 'population': 10}}}

>>> for country, cities in x.iteritems():
        y[country] = {attr:0 for attr in attributes}
        for city, attributes in cities.iteritems():
            for attribute, value in attributes.iteritems():
                y[country][attribute] += value

>>> y
{'china': {'gdp': 110, 'murders': 1100, 'population': 11}, 'usa': {'gdp': 300, 'murders': 3000, 'population': 30}}

How about something like:

for country in x:
    total_population = 0
    for city, population in x[country].iteritems():
        total_population += population
    y[country] = total_population

What you want is a new dictionary whose key is the original key and whose value is the sum of the values in the current value-dictionary.

y = dict((c[0], sum(c[1].values())) for c in x.iteritems())