Sum nested lists based on condition in Python

Question

I have a nested list looking like this:

[[\'Vienna\',\'2012\', 890,503,70],[\'London\',\'2014\', 5400, 879,78],
 [\'London\',\'2014\',4800,70,90],[\'Bern\',\'201         


        

Answer 1

You could construct a result dict where key is tuple of first two items in the original lists and value is list of numbers. Every time you add value to dict you could use get to either return existing element or given default value, in this case empty list.

Once you have the existing list and list to add you can use zip_longest with fillvalue to get numbers to sum from both lists. zip_longest returns tuples of length 2 containing one number from each list. In case one list is longer than other fillvalue is used as default so this will also work in case lists have different lengths. Finally list comprehension could used to sum each item for a new value:

from itertools import zip_longest

l = [
    ['Vienna','2012', 890,503,70],['London','2014', 5400, 879,78],
    ['London','2014',4800,70,90],['Bern','2013',300,450,678],
    ['Vienna','2013', 700,850,90], ['Bern','2013',500,700,90]
]

res = {}
for x in l:
    key = tuple(x[:2])
    res[key] = [i + j for i, j in zip_longest(res.get(key, []), x[2:], fillvalue=0)]

print(res)

Output:

{('Vienna', '2013'): [700, 850, 90], ('London', '2014'): [10200, 949, 168], 
 ('Vienna', '2012'): [890, 503, 70], ('Bern', '2013'): [800, 1150, 768]}  

If you want to sort the cities alphabetically and years latest first you could pass custom key to sorted:

for item in sorted(res.items(), key=lambda x: (x[0][0], -int(x[0][1]))):
    print(item)

Output:

(('Bern', '2013'), [800, 1150, 768])
(('London', '2014'), [10200, 949, 168])
(('Vienna', '2013'), [700, 850, 90])
(('Vienna', '2012'), [890, 503, 70])