defaultdict

A common use case for dictionaries is to hold a count of the occurrences of a key in some other object, for example, words in a file. In this case, one needs to deal with missing keys:

>>> L = list('abcaba') >>> D = dict() >>> for k in L: ... try: ... D[k] += 1 ... except KeyError: ... D[k] = 1 ... >>> D {'a': 3, 'c': 1, 'b': 2}

Another solution is to use the setdefault method of dicts:

>>> D = dict() >>> for k in L: ... D.setdefault(k,0) ... D[k] += 1 ... 0 0 0 1 1 2 >>> D {'a': 3, 'c': 1, 'b': 2}

A better way (certainly better than this second approach) is to use a defaultdict:

>>> from collections import defaultdict >>> D = defaultdict(int) >>> for k in L: ... D[k] += 1 ... >>> D defaultdict(, {'a': 3, 'c': 1, 'b': 2})

Some of the magic involves __missing__:

>>> 'd' in D False >>> D.__missing__('d') 0 >>> 'd' in D True >>> D defaultdict(, {'a': 3, 'c': 1, 'b': 2, 'd': 0})

For more you can go to the docs or here or here, or just do this:

>>> print defaultdict.__missing__.__doc__ __missing__(key) # Called by __getitem__ for missing key; pseudo-code: if self.default_factory is None: raise KeyError((key,)) self[key] = value = self.default_factory() return value

[ UPDATE: The collections.Counter class was mentioned in a comment. Don't know why I didn't mention it in this post, I've used it in many other posts here. If it's convenient to convert the data to a list of keys, that's the way to go. ]