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Function: require_once

name Punditsdkoslkdosdkoskdo

Appending turns my list to NoneType

In Python Shell, I entered:

aList = ['a', 'b', 'c', 'd']  
for i in aList:  

and got


but when I tried:

aList = ['a', 'b', 'c', 'd']  
aList = aList.append('e')  
for i in aList:  

and got

Traceback (most recent call last):  
  File "<pyshell#22>", line 1, in <module>  
    for i in aList:  
TypeError: 'NoneType' object is not iterable  

Does anyone know what's going on? How can I fix/get around it?

list.append is a method that modifies the existing list. It doesn't return a new list -- it returns None, like most methods that modify the list. Simply do aList.append('e') and your list will get the element appended.

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    • And since it doesn't return anything, you're setting aList to None if you do do the assignment, which is why you're getting the error.
    • @kindall: "doesn't return anything" should be "in effect, it's the same is if the method doesn't have a return statement and returns None implicitly". And. "Methods which mutate an object almost never return a value, pop is the notable exception."

Generally, what you want is the accepted answer. But if you want the behavior of overriding the value and creating a new list (which is reasonable in some cases^), what you could do instead is use the "splat operator", also known as list unpacking:

aList = [*aList, 'e']
#: ['a', 'b', 'c', 'd', 'e']

Or, if you need to support python 2, use the + operator:

aList = aList + ['e']
#: ['a', 'b', 'c', 'd', 'e']

^ There are many cases where you want to avoid the side effects of mutating with .append(). For one, imagine you want to append something to a list you've taken as a function argument. Whoever is using the function probably doesn't expect that the list they provided is going to be changed. Using something like this keeps your function "pure" without "side effects".

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