Question

Hi want to have a ordered dictionary with keys having list of values.

from below code i could able to get a dictionary with list of keys but ordered of insertion is missing.

from collections import defaultdict

keys=['blk','pri','ani']
vals1=['blocking','primary','anim']
vals2=['S1','S2','S3']
dic = defaultdict(list)

i=0

for key in keys:
    dic[key].append(vals1[i])
    dic[key].append(vals2[i])

    i += 1

print dic

i get the following result

defaultdict(<type 'list'>, {'pri': ['primary', 'S2'], 'ani': ['anim', 'S3'], 'blk': ['blocking', 'S1']})

here i lost insert order.

I know defaultdict object in Python are unordered by definition.

And i know we need to Use OrderedDict if you need the order in which values were inserted (it’s available in Python 2.7 and 3.x)

So changed my code as below

from below code i could able to get what i need.

from collections import defaultdict,OrderedDict

keys=['blk','pri','ani']
vals1=['blocking','primary','anim']
vals2=['S1','S2','S3']
dic = OrderedDict(defaultdict(list))

i=0

for key in keys:
    dic[key].append(vals1[i])
    dic[key].append(vals2[i])

    i += 1

print dic

and now i get the below error

Traceback (most recent call last):
  File "Dict.py", line 18, in <module>
    dic[key].append(vals1[i])
KeyError: 'blk' 

Can any one tell me how to get what i am trying.

Answer 1

If you really want the “Ordered” and “default” behavior, I think I would create a custom dictionary class to handle everything for me:

from collections import OrderedDict

class DefaultListOrderedDict(OrderedDict):
    def __missing__(self,k):
        self[k] = []
        return self[k]

keys=['blk','pri','ani']
vals1=['blocking','primary','anim']
vals2=['S1','S2','S3']
dic = DefaultListOrderedDict()
for i,key in enumerate(keys):
    dic[key].append(vals1[i])
    dic[key].append(vals2[i])

print dic

This will behave more like the defaultdict than the other solutions, but it will retain it’s order, and it really isn’t a whole lot of code :). Finally, you could even override __init__ to allow the user to pass any “factory” they wanted (rather than hard-coding list). I’ll leave that as an exercise for the interested reader. Python is so cool.