python dict: get vs setdefault

Question

The following two expressions seem equivalent to me. Which one is preferable?

data = [('a', 1), ('b', 1), ('b', 2)]

d1 = {}
d2 = {}

for key, val in data:
    # variant 1)
    d1[key] = d1.get(key, []) + [val]
    # variant 2)
    d2.setdefault(key, []).append(val)

The results are the same but which version is better or rather more pythonic?

Personally I find version 2 harder to understand, as to me setdefault is very tricky to grasp. If I understand correctly, it looks for the value of "key" in the dictionary, if not available, enters "[]" into the dict, returns a reference to either the value or "[]" and appends "val" to that reference. While certainly smooth it is not intuitive in the least (at least to me).

To my mind, version 1 is easier to understand (if available, get the value for "key", if not, get "[]", then join with a list made up from [val] and place the result in "key"). But while more intuitive to understand, I fear this version is less performant, with all this list creating. Another disadvantage is that "d1" occurs twice in the expression which is rather error-prone. Probably there is a better implementation using get, but presently it eludes me.

My guess is that version 2, although more difficult to grasp for the inexperienced, is faster and therefore preferable. Opinions?

Answer 1

In [1]: person_dict = {}

In [2]: person_dict['liqi'] = 'LiQi'

In [3]: person_dict.setdefault('liqi', 'Liqi')
Out[3]: 'LiQi'

In [4]: person_dict.setdefault('Kim', 'kim')
Out[4]: 'kim'

In [5]: person_dict
Out[5]: {'Kim': 'kim', 'liqi': 'LiQi'}

In [8]: person_dict.get('Dim', '')
Out[8]: ''

In [5]: person_dict
Out[5]: {'Kim': 'kim', 'liqi': 'LiQi'}