Tuples and immutability

[Terry Reedy]

On 3/9/2014 6:03 PM, Gregory Ewing wrote:
> Ian Kelly wrote:
>> In my view the second one is wrong.  a += b should be understood as
>> being equivalent to a = a + b, but with the *possible* and by no means
>> guaranteed optimization that the operation may be performed in-place.
>
> This interpretation is at odds with the Language Reference,
> section 6.2.1, Augmented Assignment Statements:
>
> "An augmented assignment expression like x += 1 can be rewritten as x =
> x + 1 to
> achieve a similar, but not exactly equal effect... when possible, the
> actual operation is performed
> in-place, meaning that rather than creating a new object and assigning
> that to
> the target, the old object is modified instead."
>
> Note that it says "when possible", not "if the implementation
> feels like it".
>
>> In fact, if you read the documentation for lists, you may notice that
>> while they clearly cover the + operator and the extend method, they do
>> not explicitly document the list class's += operator.
>
> The "when possible" clause, together with the fact that lists
> are mutable, implies that it *will* be done in-place. There
> is no need to document all the in-place operators explicitly
> for every type.

The discussion of .__iop__ in the datamodel chapter was just revised 
slightly.
http://bugs.python.org/issue19953

-- 
Terry Jan Reedy