Python creating a dictionary of lists

Question

You can use defaultdict:

>>> from collections import defaultdict
>>> d = defaultdict(list)
>>> a = ['1', '2']
>>> 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'])]

You can build it with list comprehension like this:

>>> dict((i, range(int(i), int(i) + 2)) for i in ['1', '2'])
{'1': [1, 2], '2': [2, 3]}

And for the second part of your question use defaultdict

>>> 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)

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

You can use setdefault:

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

print d  # prints {1: ['1'], 2: ['1', '2'], 3: ['2']}

The rather oddly-named setdefault function says "Get the value with this key, or if that key isn't there, add this value and then return it."

As others have rightly pointed out, defaultdict is a better and more modern choice. setdefault is still useful in older versions of Python (prior to 2.5).

Personally, I just use JSON to convert things to strings and back. Strings I understand.

import json
s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
mydict = {}
hash = json.dumps(s)
mydict[hash] = "whatever"
print mydict
#{'[["yellow", 1], ["blue", 2], ["yellow", 3], ["blue", 4], ["red", 1]]': 'whatever'}

Your question has already been answered, but IIRC you can replace lines like:

if d.has_key(scope_item):

with:

if scope_item in d:

That is, d references d.keys() in that construction. Sometimes defaultdict isn't the best option (for example, if you want to execute multiple lines of code after the else associated with the above if), and I find the in syntax easier to read.