<s>[INST] Python creating a dictionary of lists I want to create a dictionary whose values are lists.  For example: 
{
  1: ['1'],
  2: ['1','2'],
  3: ['2']
}

If I do:
d = dict()
a = ['1', '2']
for i in a:
    for j in range(int(i), int(i) + 2): 
        d[j].append(i)

I get a KeyError, because d[...] isn't a list.  In this case, I can add the following code after the assignment of a to initialize the dictionary. 
for x in range(1, 4):
    d[x] = list()

Is there a better way to do this?  Lets say I don't know the keys I am going to need until I am in the second for loop.  For example:
class relation:
    scope_list = list()
...
d = dict()
for relation in relation_list:
    for scope_item in relation.scope_list:
        d[scope_item].append(relation)

An alternative would then be replacing 
d[scope_item].append(relation)

with 
if d.has_key(scope_item):
    d[scope_item].append(relation)
else:
    d[scope_item] = [relation,]

What is the best way to handle this?  Ideally, appending would "just work".  Is there some way to express that I want a dictionary of empty lists, even if I don't know every key when I first create the list?
 [/INST] You can use defaultdict:
>>> from collections import defaultdict
>>> d = defaultdict(list)
>>> for i in a:
...   for j in range(int(i), int(i) + 2):
...     d[j].append(i)
...
>>> d
defaultdict(<type 'list'>, {1: ['1'], 2: ['1', '2'], 3: ['2']})
>>> d.items()
[(1, ['1']), (2, ['1', '2']), (3, ['2'])]

 </s>