为什么当我使用 list.extend 调用它时扩展方法不起作用？

Question

When i call the extend method of list in python using list.extend(list_object) i get an error. Why is this happening? Instead of declaring an object of list class i am directly calling the extend using list.extend. the following is the code which i wrote.

l=list()
l2=[1,2,3,4]
print(list.extend(l2))# throws an error.I was under the impression that this is same as the below statement
print(l.extend(l2)) #doesnt throw an error

Also,

class student():
        a=1

s=student()
s.a #prints 1.
student.a #also prints 1. Here the previous error isnt coming.

Why is this so?

Answer 1

Both cases are not the same.

In the first case, you're calling a method (a callable attribute) of a builtin class via the class and then an instance of the class. In the second case, you're accessing a non callable attribute of the class, via the instance and then via the class. Methods are callable and in order to call them, you need to respect the signature of the method, unlike the second case.

To use extend via the list class itself, you need to pass a list instance as first argument, as vanilla methods in Python usually require an instance as first argument:

list.extend(l, l2) # extends l with l2

Note that the instance is implicitly passed when you call extend via the instance in l.extend(l2) .