python中，直接print（列表.函数）会输出NONE？

再python中，例如下面一段代码是可以实现目标的，输入：
lucky_numbers=[18,99,5,33,88,23,16]
friends=["Kevin","Karen","Jim","Oscar","Tim"]

friends.extend(lucky_numbers)

print(friends)

输出：
C:\Users\Administrator\PycharmProjects\Giraffe\venv\Scripts\python.exe C:/Users/Administrator/PycharmProjects/Giraffe/app.py
['Kevin', 'Karen', 'Jim', 'Oscar', 'Tim', 18, 99, 5, 33, 88, 23, 16]

Process finished with exit code 0
（这是正确的结果）

但是，若代码是这样的：
lucky_numbers=[18,99,5,33,88,23,16]
friends=["Kevin","Karen","Jim","Oscar","Tim"]

print(friends.extend(lucky_numbers))

就会输出结果：
C:\Users\Administrator\PycharmProjects\Giraffe\venv\Scripts\python.exe C:/Users/Administrator/PycharmProjects/Giraffe/app.py
None

Process finished with exit code 0

请问这是什么原理？