为什么当我扩展列表时，它的类型是“NoneType”？

Question

I am looking to extend a list and have the following

ListA = list(x)
ListB = list(y)
ListC = ListA.extend(ListB)

This gives ListC as having <type 'NoneType'> and I am wondering why this isn't just a list as well.

Thanks!

Answer 1

extend , like many other list operations, operates in-place and returns None. ListA is modified with the extra elements.

Answer 2

You did not define ListC as a list type.

As you know, list() is a function; which generates a list type.

list.extend does not. Instead, it modifies the given list.

Look at your code again.

ListA = list(x) ## You defined ListA as a list type; with list(x). 

ListB = list(y) ## You defined ListB as a list type; with list(y). 

ListC = ListA.extend(ListB) ## ListC is not defined as a list type. 

Therefore, ListC returns NoneType.

It is good that you noticed the difference between + and list.extend(). Keep it up and good luck.