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Python Introducing Lists Meet Lists Addition

Posted by juanmendoza10
juanmendoza10
4,839 Points

+= operator for list concatenation instead?

I was playing with the REPL and I discovered you can extend lists using the += operator as well. Is there any detriment to doing this? Any unexpected bugs I could run into in the future?

1 Answer

Steven Parker
Steven Parker
231,275 Points
Steven Parker
Steven Parker
231,275 Points

You cannot use the compounding assignment operator on a non-local variable (one that is not local to a function and also not global):

def test():
    nonloc = [1, 2, 3]

    def extendit():
        nonloc.extend([4])

    def compoundit():
        nonloc += [5]

    extendit()    # this will work
    compoundit()  # but this will fail

test()

You'll get an "UnboundLocalError" (local variable referenced before assignment).

juanmendoza10
juanmendoza10
4,839 Points

Ah, I see. So if I'm understanding this correctly, the scope of the compounding assignment operator is only within a code block. Could you explain why?

Steven Parker
Steven Parker
231,275 Points
Steven Parker
Steven Parker
231,275 Points

It's not a scope issue, it has to do with how the system accesses the references. With that operator it appears that you are attempting to reference a new variable before it is created.

The intended variable is actually available to the function, but you have to declare it before accessing it:

    def compoundit():
        nonlocal nonloc  # adding this declaration will make it work
        nonloc += [5]

But the point is that you don't encounter these complications when using "extend".