Confusing simple python list behavior with +=

Question

list4=[7,8]
def proc3(mylist):
    mylist+=[9]


print list4
proc3(list4)
print list4

Why does this code produce the answers [7,8] and [7,8,9]? I expected [7,8] and [7,8]. Doesn't mylist+=[9] concatenate the number 9 and create a new list as a local variable only? Why does the "print list4" after the running proc3(list4), but outside the procedure, not result in the original list? I must be missing something obvious here. Thanks in advance for any help.

Even though you are correct that `proc3` has a different reference to the `list` object than the one in your global namespace, the `+=` operator *mutates* the list. You're altering the object itself, it doesn't matter that you're doing it with a second reference. — roippi, Jan 03 '14 at 05:12
Check this answer which explains everything you need to know for pass by value and pass by reference in python http://stackoverflow.com/questions/986006/python-how-do-i-pass-a-variable-by-reference — Aks, Jan 03 '14 at 05:12

score 3 · Accepted Answer · 2014-01-03T05:15:25.173

3

+= does not return a new list object. Instead, it modifies the original list object in-place.

In other words, this line inside proc3:

mylist+=[9]

is modifying the list object referenced by mylist. As a result, it is no longer [7, 8] but [7, 8, 9].

edited Jan 03 '14 at 05:15

answered Jan 03 '14 at 05:09

So does the += operator act similarly to mylist.append(9) which mutates the list? I thought it was the same as mylist=mylist+[9], which points to a new concatenated list... – user3150100 Jan 03 '14 at 05:16
One more thing. It's not exactly the same as .append, because mylist.append([9,10]) creates [7,8,[9,10]], whereas mylist+=[9,10] creates [7,8,9,10], right? Thanks for the help, this is quite subtle for such a simple operation :-). – user3150100 Jan 03 '14 at 05:59
@iCodez Actually, `+=` works exactly like `mylist.extend`. If it were like `mylist.append` then `mylist += [9]` would result in `[7, 8, [9]]`. It's an important distinction, especially if someone were to do something like `mylist += 'hello'` and would get the unexpected result of `[7, 8, 'h', 'e', 'l', 'l', 'o']` – SethMMorton Jan 03 '14 at 07:18
@user3150100 Ha, yes, I should have read your comment first. `+=` is a shortcut for `list.extend`. – SethMMorton Jan 03 '14 at 07:19
@SethMMorton - Yes, my mistake (I'll remove my previous comment so that there is no future confusion :). When the OP said `list.append`, I focused on it being an in-place operation like `+=`. But `list.extend` is a lot more accurate. – Jan 03 '14 at 14:29

score 1 · Answer 2 · edited May 23 '17 at 12:14

Quoting this answer in another post:

Everything is a reference in Python. If you wish to avoid that behavior you would have to create a new copy of the original with list(). If the list contains more references, you'd need to use deepcopy():

Let's:

list4 = [7, 8]
def proc3(mylist):
    print id(list4)
    mylist += [9]
    print id(list4)


print list4
print id(list4)
proc3(list4)
print id(list4)
print list4

This would print:

So as you can see, it's the same list in every moment.

score 0 · Answer 3 · answered Jan 03 '14 at 05:10

0

the += operator modifies the object. Notice that

x += y

is much different from

x = x + y

(check with your test code)

answered Jan 03 '14 at 05:10

Emanuele Paolini

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Confusing simple python list behavior with +=

3 Answers3