Default Dictionary

"The standard dictionary includes the method setdefault() for retrieving a value and establishing a default if the value does not exist. By contrast, defaultdict lets the caller specify the default(value to be returned) up front when the container is initialized."

Import

from collections import defaultdict

List

> list_dict = defaultdict(list)
> s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
> for k, v in s:
      list_dict[k].append(v)
> sorted(list_dict.items())
[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

When each key is encountered for the first time, it is not already in the mapping; so an entry is automatically created using the default_factory function which returns an empty list. The list.append() operation then attaches the value to the new list. When keys are encountered again, the look-up proceeds normally (returning the list for that key) and the list.append() operation adds another value to the list.

Int

Setting the default_factory to int makes the defaultdict useful for counting (like a bag or multiset in other languages) When a letter is first encountered, it is missing from the mapping, so the default_factory function calls int() to supply a default count of zero. The increment operation then builds up the count for each letter.

> int_dict = defaultdict(int)
> s = 'mississippi'
> int_dict = defaultdict(int)
> for k in s:
      int_dict[k] += 1

> sorted(d.items())
[('i', 4), ('m', 1), ('p', 2), ('s', 4)]```

Function

> def foo():
>   return 'default value'
> func_dict = defaultdict(foo)

Set

Setting the default_factory to set makes the defaultdict useful for building a dictionary of sets:

> s = [('red', 1), ('blue', 2), ('red', 3), ('blue', 4), ('red', 1), ('blue', 4)]
> d = defaultdict(set)
> for k, v in s:
      d[k].add(v)

> sorted(d.items())
[('blue', {2, 4}), ('red', {1, 3})]