On 3/11/2014 10:01 PM, Rick Johnson wrote:
On Thursday, February 27, 2014 4:18:01 PM UTC-6, Ian wrote:
x += y is meant to be equivalent, except possibly in-place and
more efficient, than x = x + y.
The manual actually says "An augmented assignment expression like x += 1
can be rewritten as x = x + 1 to achieve a similar, but not exactly
equal effect. In the augmented version, x is only evaluated once. Also,
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.
In an ideal world, the speed of these two codes should be the same,
Nope, 'similar' is not 'equivalent'. Evaluating x twice instead of once
and possibly allocating a new object versus not take extra time. In a
statement like "x.y.z[3*n+m] += 1", calculating the target dominates the
time to increment, so this form should be nearly twice as fast.
--
Terry Jan Reedy
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