appending values to dictionary in for loop

Question

Just cant get this working. Any help is highly appreciated.

dict = {}
for n in n1:
    if # condition #
        dict[key] = []
        dict[key].append(value)
        print dict

This is printing something like this

{'k1':['v1']} and {'k1':['v2']}

I have few other nested for loops down this code, which will be using this dict and the dict has only the latest key value pairs i.e. {'k1':'v2'}

I am looking for something like {'k1':['v1','v2']}

Please suggest a solution without using setdefault

Answer 1

You can also check key existence before assign.

dict = {}
for n in n1:
    if # condition #
        if key not in dict:
            dict[key] = []
        dict[key].append(value)
        print dict

Answer 2

Give collections.defaultdict a try:

#example below is in the docs.
from collections import defaultdict

s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
d = defaultdict(list)
for k, v in s:
    d[k].append(v)

print(sorted(d.items()))
[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

the d = defaultdict(list) line is setting the keys values to be an empty dictionary by default and appending the value to the list in the loop.

Answer 3

The problem with the code is it creates an empty list for 'key' each time the for loop runs. You need just one improvement in the code:

dict = {}
dict[key] = []
for n in n1:
   if # condition #
       dict[key].append(value)
       print dict