On Tue, May 21, 2013 at 12:35 AM, r.david.murray <python-checkins@python.org> wrote: Yay for having this in the FAQ, but...

+If you wrote:: + + >>> a_tuple = (1, 2) + >>> a_tuple[0] += 1 + Traceback (most recent call last): + ... + TypeError: 'tuple' object does not support item assignment + +The reason for the exception should be immediately clear: ``1`` is added to the +object ``a_tuple[0]`` points to (``1``), producing the result object, ``2``, +but when we attempt to assign the result of the computation, ``2``, to element +``0`` of the tuple, we get an error because we can't change what an element of +a tuple points to. + +Under the covers, what this augmented assignment statement is doing is +approximately this:: + + >>> result = a_tuple[0].__iadd__(1) + >>> a_tuple[0] = result + Traceback (most recent call last): + ... + TypeError: 'tuple' object does not support item assignment

For the immutable case, this expansion is incorrect:

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...

hasattr(0, "__iadd__") False

With immutable objects, += almost always expands to:

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result = a_tuple[0] + 1 a_tuple[0] = result

(For containers that support binary operators, the presence of the relevant __i*__ methods is actually a reasonable heuristic for distinguishing the mutable and immutable versions) Cheers, Nick. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia