python: how to add a collections.defaultdict(int) to another?

Question

Something like statement [4] is what I'd like...

(obtained, clumsily & non-optimally, via statements [5] and [6])

In [1]: from collections import defaultdict

In [2]: d1 = defaultdict(int, dict(a=1, b=2, c=3))

In [3]: d2 = defaultdict(int, dict(a=10, c=30, d=40))

In [4]: d1 |= d2
TypeError: unsupported operand type(s) for |=: 'collections.defaultdict' and 'collections.defaultdict'

In [5]: def default_dict_add(d1, key, val): 
            d1[key] += val

In [6]: [default_dict_add(d1, k, d2[k]) for k in d2.keys()]
Out[6]: 
[None, None, None]

In [7]: d1
defaultdict(int, {'a': 11, 'b': 2, 'c': 33, 'd': 40})

Similar to what you can do with sets (statement # [44]

In [42]: s1 = {1, 2, 3}
s1 = {1, 2, 3}

In [43]: s2 = {10, 30, 40}
s2 = {10, 30, 40}

In [44]: s1 |= s2

In [45]: s1

Out[45]: 
{1, 2, 3, 10, 30, 40}

score 0 · Accepted Answer · answered Apr 04 '17 at 02:54

0

It seems like what you're doing would be better suited for a Counter, which is in the same module as defaultdict.

d1 = Counter(dict(a=1, b=2, c=3))
d2 = Counter(dict(a=10, c=30, d=40))
d1 + d2
# Counter({'d': 40, 'c': 33, 'a': 11, 'b': 2})

answered Apr 04 '17 at 02:54

Arya McCarthy

8,554
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1

You can also use `d1 += d2` if you want to modify `d1` as in your example. – Arya McCarthy Apr 04 '17 at 02:56

score 0 · Answer 2 · answered Apr 04 '17 at 02:55

You can use Counter:

>>> from collections import Counter
>>> c1 = Counter(d1)
>>> c2 = Counter(d2)
>>> c1 + c2
Counter({'d': 40, 'c': 33, 'a': 11, 'b': 2})

Or:

>>> {k: d1.get(k, 0) + d2.get(k, 0) for k in set(list(d1.keys()) + list(d2.keys()))}
{'a': 11, 'c': 33, 'b': 2, 'd': 40}

python: how to add a collections.defaultdict(int) to another?

2 Answers2