collections

The purpose of defaultdict in Python (from the collections module) is to simplify handling missing keys in a dictionary by automatically assigning them a default value type when they’re first accessed.

defaultdict saves you from writing repetitive code to check and initialize missing keys, making your code cleaner and safer.

Common Use Cases

• Grouping items:

from collections import defaultdict
grouped = defaultdict(list)
for key, value in some_data:
    grouped[key].append(value)

• Counting:

from collections import defaultdict
count = defaultdict(int)
for item in data:
    count[item] += 1

• Set-based accumulations:

from collections import defaultdict
unique = defaultdict(set)
unique['a'].add('x')

Examples

Example 1: Grouping

Without  defaultdict If you try to access a key that doesn’t exist in a normal dictionary, you’ll get a KeyError.

d = {}
d['a']  # KeyError

You often have to use a pattern like:

# Pattern 1
d = {}
if 'a' not in d:
    d['a'] = []
d['a'].append(1)
 
# Pattern 2
d = {}
d['a'] = d.get('a',[])
d['a'].append(1)

With  defaultdict You can define what the default value should be for missing keys using a factory function (like list, int, set, etc.):

from collections import defaultdict
 
d = defaultdict(list)
d['a'].append(1)  # No KeyError, creates d['a'] = [] automatically

Example 2: Counting

data = 'mississippi'
count_d = defaultdict(int)
for letter in data:
  count_d[letter] += 1
count_d.items()

dict_items([('m', 1), ('i', 4), ('s', 4), ('p', 2)])