put lists inside dictionary

Question

I want to put lists inside a dictionary. I want do like this:

trail = {[]}

for i in states:
    for j in states:
        if(max_val < temp_V[j][i]):
            max_val = temp_V[j][i]
            trail[i].append(j)

But I am getting an error at trail = {[]}. I'm new to Python. How can I do this?

`{[]}` is a set not dictionary, sets can't contain unhashable values. — Ashwini Chaudhary, May 29 '13 at 09:49
possible duplicate: http://stackoverflow.com/questions/960733/python-creating-a-dictionary-of-lists — aldeb, May 29 '13 at 09:50

score 4 · Accepted Answer · answered May 29 '13 at 09:49

Use defaultdict

from collections import defaultdict
trail = defaultdict(list)

It will construct an empty list for any key when you try to access it for the first time so

trail[i].append(j)

will work as expectd. As for:

trail = {[]}

You need to have a keys and corresponding values for a valid dictionary literal eg.

{'a': 1, 'b': 2}

{[]} trys to makes a set instead eg.

>>> {1}
set([1])

but sets only support hashable items, see hash table, and a list (which can be modified) is therefore not hashable.

put lists inside dictionary

1 Answers1