[issue16701] Docs missing the behavior of += (in-place add) for lists.

[Martin Panter]

Martin Panter added the comment:

The “s += t” operation assigns the result back to s. So it could involve an extra __setattr__()/__setitem__() call, or an exception trying to modify a tuple item, etc. The extend() and slice assignment versions don’t have this extra stage.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue16701>
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