Adding missing keys in dictionary in Python

Question

I have a list of dictionaries:

L = [{0:1,1:7,2:3,4:8},{0:3,2:6},{1:2,4:6}....{0:2,3:2}]. 

As you can see, the dictionaries have different length. What I need is to add missing keys:values to every dictionary to make them being with the same length:

L1 = [{0:1,1:7,2:3,4:8},{0:3,1:0,2:6,3:0,4:0},{0:0, 1:2,3:0,4:6}....{0:2,1:0,2:0,3:2,4:0}], 

Means to add zeros for missing values. The maximum length isn't given in advance, so one may get it only iterating through the list.

I tried to make something with defaultdicts, like L1 = defaultdict(L) but it seems I don't understand properly how does it work.

Answer 1

a bit of caution: changes L

>>> allkeys = frozenset().union(*L)
>>> for i in L:
...    for j in allkeys:
...        if j not in i:
...            i[j]=0

>>> L
[{0: 1, 1: 7, 2: 3, 3: 0, 4: 8}, {0: 3, 1: 0, 2: 6, 3: 0, 4: 0}, {0: 0, 1: 2, 2:
 0, 3: 0, 4: 6}, {0: 2, 1: 0, 2: 0, 3: 2, 4: 0}]

Answer 2

You'll have to make two passes: 1 to get the union of all keys, and another to add the missing keys:

max_key = max(max(d) for d in L)
empty = dict.fromkeys(range(max_key + 1), 0)
L1 = [dict(empty, **d) for d in L]

This uses an 'empty' dictionary as a base to quickly produce all keys; a new copy of this dictionary plus an original dictionary produces the output you want.

Note that this assumes your keys are always sequential. If they are not, you can produce the union of all existing keys instead:

empty = dict.fromkeys(set().union(*L), 0)
L1 = [dict(empty, **d) for d in L]

Demo:

>>> L = [{0: 1, 1: 7, 2: 3, 4: 8}, {0: 3, 2: 6}, {1: 2, 4: 6}, {0: 2, 3: 2}]
>>> max_key = max(max(d) for d in L)
>>> empty = dict.fromkeys(range(max_key + 1), 0)
>>> [dict(empty, **d) for d in L]
[{0: 1, 1: 7, 2: 3, 3: 0, 4: 8}, {0: 3, 1: 0, 2: 6, 3: 0, 4: 0}, {0: 0, 1: 2, 2: 0, 3: 0, 4: 6}, {0: 2, 1: 0, 2: 0, 3: 2, 4: 0}]

or the set approach:

>>> empty = dict.fromkeys(set().union(*L), 0)
>>> [dict(empty, **d) for d in L]
[{0: 1, 1: 7, 2: 3, 3: 0, 4: 8}, {0: 3, 1: 0, 2: 6, 3: 0, 4: 0}, {0: 0, 1: 2, 2: 0, 3: 0, 4: 6}, {0: 2, 1: 0, 2: 0, 3: 2, 4: 0}]

The above approach to merge two dictionaries into a new one with dict(d1, **d2) always works in Python 2. In Python 3 additional constraints have been set on what kind of keys you can use this trick with; only string keys are allowed for the second dictionary. For this example, where you have numeric keys, but you can use dictionary unpacking instead:

{**empty, **d}  # Python 3 dictionary unpacking

That'll work in Python 3.5 and newer.