Question

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What does plus equals (+=) do in Python?

I noticed a strange problem:

    l1 = ['1', '2', '3']
    l2 = l1

    item = l2.pop(0)
    # the pop operation will effect l1
    print l1

    l2 = l2 + [item]
    # why "l2 = l2 + [item]" does't effect l1 while "l2 += [item]" does.
    print l2
    print l1

The output is:

['2', '3']
['2', '3', '1']
['2', '3']

But if i change l2 = l2 + [item] into l2 += [item], the output will be:

['2', '3']
['2', '3', '1']
['2', '3', '1']

Answer 1

+ and += are different operators woth different internal meaning as described here.

l2 = l1 + x calls l2 = l1.__add__(x), if that doesn’t work it calls x.__radd__(l1). Both should return a new object forming the result of the operation, independent from the old one, thus not affecting l1.

l2 += x calls l2 = l2.__iadd__(x) (“augmented assignment”), and only if this doesn’t work, falls back to l2 = l2 + x as described above.

With numbers, both are the same, because they are immutable and thus cannot be modified with +=, while on lists, + returns a new object while += modifies the left hand side one.

As the object behind l2 is modified and l1 refers the same object, you notice the change on l1 as well.