python defaultdict: デフォルト値を自由に設定

lambdaを使えば自由に初期値を設定できます。

In [1]: from collections import defaultdict
In [2]: d = defaultdict(int)
In [3]: d['a']
Out[3]: 0
In [4]: d['a'] += 1
In [5]: d['a']
Out[5]: 1
In [6]: d = defaultdict(lambda: int(5))
In [7]: d['a']
Out[7]: 5
In [8]: d['a'] += 1
In [9]: d['a']
Out[9]: 6
In [10]: d = defaultdict(lambda: [1,2,3,4,5])
In [11]: d['a']
Out[11]: [1, 2, 3, 4, 5]
In [12]: d['a'].append(6)
In [13]: d['a']
Out[13]: [1, 2, 3, 4, 5, 6]