defaultdict with a parameter to the class constructor

>>> def f():
        return 1
>>> a = defaultdict(f)
>>> a[1]
1

Here is an other implementation using lambda expression (from kindall):

>>> a = defaultdict(lambda: 1)

defaultdict(lambda: 1)

eg

>>> from collections import defaultdict
>>> a = defaultdict(lambda: 1)
>>> a["foo"] += 1
>>> a["foo"]
2