Question

Possible Duplicate:
“Least Astonishment” in Python: The Mutable Default Argument
List extending strange behaviour
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Let’s say I have this function:

def a(b=[]):
    b += [1]
    print b

Calling it yields this result:

>>> a()
[1]
>>> a()
[1, 1]
>>> a()
[1, 1, 1]

When I change b += [1] to b = b + [1], the behavior of the function changes:

>>> a()
[1]
>>> a()
[1]
>>> a()
[1]

How does b = b + [1] differ from b += [1]? Why does this happen?

Answer 1

In Python there is no guarantee that a += b does the same thing as a = a + b.

For lists, someList += otherList modifies someList in place, basically equivalent to someList.extend(otherList), and then rebinds the name someList to that same list. someList = someList + otherList, on the other hand, constructs a new list by concatenating the two lists, and binds the name someList to that new list.

This means that, with +=, the name winds up pointing to the same object it already was pointing to, while with +, it points to a new object. Since function defaults are only evaluated once (see this much-cited question), this means that with += the operations pile up because they all modify the same original object (the default argument).