Often you need to store data (counters, sums, lists, etc.) about some entities. A regular Python dictionary will suffice with one caveat: before adding information regarding a particular entity you need to check if the corresponding key exists in the dictionary, otherwise you will get a KeyError. Do not know about you, but I have written similar code many times:

dct = {}

if key not in dct:
    dct['key'] = 0

dct['key'] += 1

A better way

While it is perfectly workable, it does not add to readability. There is a better, more pythonic solution – defaultdict (documentation ).

It is a subclass of regular dict which accepts a default_factory which is run every time you try to access a non-existing item in the dict. This way the code above can be simplified as follows:

from collections import defaultdict

dct = defaultdict(int)
dct['key'] += 1

Notes

  1. The default mechanism is a factory, not a value. I.e. it must be callable, for instance: built-in type, named, or lambda function.

  2. If you initialize a defaultdict without a factory or with None it will act as a regular dict, including throwing a KeyError when accessing a non-existent key.

  3. default_factory is a property and you can change it after initializing the dict:

    from collections import defaultdict

dct = defaultdict(int)
dct['int'] += 1

dct.default_factory = list
dct['list'].append(0) 
# defaultdict(<class 'list'>, {'int': 1, 'list': [0]})

Examples

from collections import defaultdict

defaultdict(int)  # int evaluates to 0
defaultdict(list)  # Empty list 
defaultdict(lambda: 10)  # Arbitrary lambda function
defaultdict(your_function)  # Arbitrary named function