33

In Python Shell, I entered: 

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

and got 

a  
b  
c  
d

but when I tried: 

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

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?

SilentGhost
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user460847
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3 Answers3

49

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.

Thomas Wouters
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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 Oct 01 '10 at 15:48
  • All functions return something :) – Thomas Wouters Oct 01 '10 at 15:48
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    true - but sometimes that *something* is `None`. But is `None` really something? My head hurts! – Mark Ransom Oct 01 '10 at 15:58
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    @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." – S.Lott Oct 01 '10 at 16:08
5

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".

SCB
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4

Delete your second line aList = aList.append('e') and use only aList.append("e"), this should get rid of that problem.

Werner
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aletelecomm
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