Understand Python defaultdict(list): A Beginner Tutorial

By | June 10, 2021

Python defaultdict() will allow us to create a default value for dict data type. Meanwhile, you may find the code:

result = defaultdict(list)

In this tutorial, we will use some examples to help you understand it.

Python defaultdict()

Python defaultdict() can create a default diction using a factory function.

dict =defaultdict( factory_function)

The factory_function can be list, set, str et al.

When we read an nonexistent key, it will read a default value by factory_function. Here is an example:

from collections import defaultdict

dict1 = defaultdict(int)
dict2 = defaultdict(set)
dict3 = defaultdict(str)
dict4 = defaultdict(list)
dict1[2] ='two'

print(dict1[1])
print(dict2[1])
print(dict3[1])
print(dict4[1])

Run this code, you will see result:

0
set()

[]

What is defaultdict(list)?

defaultdict(list) means will create a diction data with default value [].

We can append a new value to a key directly.

Here is an example:

from collections import defaultdict

dict = defaultdict(list)
dict['name'].append("Tom")
dict['name'].append("Lily")
dict['name'].append("Kate")

print(dict)

Run this code, you will see:

defaultdict(<class 'list'>, {'name': ['Tom', 'Lily', 'Kate']})

In this code, we append new name to dict[‘name’] without checking key ‘name‘ is existing or not.

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