Python: updating a list as a dictionary value

Question

I want to have lists as values of a dictionary and want to ability to append to those list when there is a match in key.

I understand I can do it when the list is predefined as follows:

hashmap = {}
k = [1,2,3]
val = ['a','b','c']
for i in k:
    hashmap[i]= val
for j in hashmap.keys():
    print(hashmap[j])

But what if the contents of the val list is not defined. How do I declare it runtime and append to those list?

I don't understand your question. If val is not defined, how do you expect to use `val`? Also note, you do not have *lists* here, you have a **single list**. — juanpa.arrivillaga, Jan 29 '18 at 22:59
Kind of sounds like this https://stackoverflow.com/questions/26367812/appending-to-list-in-python-dictionary — Orenshi, Jan 29 '18 at 23:00

score 4 · Accepted Answer · answered Jan 29 '18 at 23:00

4

Use a defaultdict to create the list if it doesn't exist:

from collections import defaultdict

dd = defaultdict(list)
dd['a'].append(1)  # create the list if it doesn't exist
print(dd)
# defaultdict(<class 'list'>, {'a': [1]})

answered Jan 29 '18 at 23:00

Ben

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Python: updating a list as a dictionary value

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