defaultdict(list)

[goungoun]

The defaultdict in Python, which is part of the collections module, is a subclass of the built-in dict class. It is particularly useful when creating a dictionary that defaults to a specific data type for missing keys. ​When the argument passed to defaultdict is list, it provides a unique utility that can enhance data organization and retrieval. Below is an explanation of its functionalities and applications:

Automatic Initialization:
When using defaultdict(list), any key that does not exist in the dictionary will automatically be initialized to an empty list when accessed. This eliminates the need to check for key existence before updating values, thus reducing boilerplate code.

Improved Code Readability:
Utilizing defaultdict with lists can improve code readability by reducing complexity. Instead of handling checks for key existence and initializing lists manually, the defaultdict simplifies this process.

from collections import defaultdict

d = defaultdict(list)

for i in range(5):
    d[i].append(i)

print("Dictionary with values as list:")
print(d)

[goungoun]

https://docs.python.org/3/library/collections.html#collections.defaultdict

Using list as the default_factory, it is easy to group a sequence of key-value pairs into a dictionary of lists:

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

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

Setting the default_factory to int makes the defaultdict useful for counting (like a bag or multiset in other languages):

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

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