Python Language Collections module collections.defaultdict


collections.defaultdict(default_factory) returns a subclass of dict that has a default value for missing keys. The argument should be a function that returns the default value when called with no arguments. If there is nothing passed, it defaults to None.

>>> state_capitals = collections.defaultdict(str)
>>> state_capitals
defaultdict(<class 'str'>, {})

returns a reference to a defaultdict that will create a string object with its default_factory method.

A typical usage of defaultdict is to use one of the builtin types such as str, int, list or dict as the default_factory, since these return empty types when called with no arguments:

>>> str()
>>> int()
>>> list

Calling the defaultdict with a key that does not exist does not produce an error as it would in a normal dictionary.

>>> state_capitals['Alaska']
>>> state_capitals
defaultdict(<class 'str'>, {'Alaska': ''})

Another example with int:

>>> fruit_counts = defaultdict(int)
>>> fruit_counts['apple'] += 2  # No errors should occur
>>> fruit_counts
default_dict(int, {'apple': 2})
>>> fruit_counts['banana']  # No errors should occur
>>> fruit_counts  # A new key is created
default_dict(int, {'apple': 2, 'banana': 0})

Normal dictionary methods work with the default dictionary

>>> state_capitals['Alabama'] = 'Montgomery'
>>> state_capitals
defaultdict(<class 'str'>, {'Alabama': 'Montgomery', 'Alaska': ''})

Using list as the default_factory will create a list for each new key.

>>> s = [('NC', 'Raleigh'), ('VA', 'Richmond'), ('WA', 'Seattle'), ('NC', 'Asheville')]
>>> dd = collections.defaultdict(list)
>>> for k, v in s:
...     dd[k].append(v)
>>> dd
defaultdict(<class 'list'>, 
    {'VA': ['Richmond'], 
     'NC': ['Raleigh', 'Asheville'], 
     'WA': ['Seattle']})