Introduction to the usage of defaultdict type in Python's collections module

defaultdict is mainly used when value needs to be initialized. For a dictionary, the key must be hashable, immutable, and unique data, and the value can be any data type. If value is a data type such as list, dict, etc., it must be initialized to empty before use. In some cases, value needs to be initialized to a special value, such as 0 or ''.

from collections import defaultdict

person_by_age = defaultdict(list)
for person in persons:
  d[person.age].append(person.name)

defaultdict is used in the same way as dict, except that a callable object x must be passed in during initialization. When a key that does not exist yet is accessed, the value will be automatically set to x(). For example, in the above example, when a person of a certain age d[person.age] is accessed for the first time, it will become list(), that is, [].

Of course, you can also use your own defined callable object, such as:

d = defaultdict(lambda: 0)
d["hello"] += 1     # 1
d["a"]         # 0

defaultdict is more efficient than dict.set_default, and is more intuitive and convenient to use.

The standard dictionary includes a method setdefault() to get a value and create a default value if the value does not exist. defaultdict initializes the container to allow the caller to specify the default value in advance.

import collections
def default_factory():
  return 'default value'
d = collections.defaultdict(default_factory, foo = 'bar')
print 'd:', d
print 'foo =>', d['foo']
print 'var =>', d['bar']

This method can be used as long as all keys have the same default value.
The result above is:

d: defaultdict(<function default_factory at 0x0201FAB0>, {'foo': 'bar'})
foo => bar
var => default value