How does collections.defaultdict work?

Question

I\'ve read the examples in python docs, but still can\'t figure out what this method means. Can somebody help? Here are two examples from the python docs

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Answer 1

My own 2¢: you can also subclass defaultdict:

class MyDict(defaultdict):
    def __missing__(self, key):
        value = [None, None]
        self[key] = value
        return value

This could come in handy for very complex cases.

Answer 2

There is a great explanation of defaultdicts here: http://ludovf.net/blog/python-collections-defaultdict/

Basically, the parameters int and list are functions that you pass. Remember that Python accepts function names as arguments. int returns 0 by default and list returns an empty list when called with parentheses.

In normal dictionaries, if in your example I try calling d[a], I will get an error (KeyError), since only keys m, s, i and p exist and key a has not been initialized. But in a defaultdict, it takes a function name as an argument, when you try to use a key that has not been initialized, it simply calls the function you passed in and assigns its return value as the value of the new key.

Answer 3

defaultdict means that if a key is not found in the dictionary, then instead of a KeyError being thrown, a new entry is created. The type of this new entry is given by the argument of defaultdict.

For example:

somedict = {}
print(somedict[3]) # KeyError

someddict = defaultdict(int)
print(someddict[3]) # print int(), thus 0