On Sun, 21 Aug 2011 09:52:23 -0700, Laurent wrote:
> I did many tests and "i = i + 1" always seems to be around 2% faster
> than "i += 1". This is no surprise as the += notation seems to be a
> syntaxic sugar layer that has to be converted to i = i + 1 anyway. Am I
> wrong in my interpretation?
It depends. If the value on the left has an __iadd__ method, that will be
called; the value will be updated in-place, so all references to that
object will be affected:
> import numpy as np
> a = np.zeros(3)
> b = a
> a
array([ 0., 0., 0.])
> b
array([ 0., 0., 0.])
> a += 1
> a
array([ 1., 1., 1.])
> b
array([ 1., 1., 1.])
If the value on the left doesn't have an __iadd__ method, then addition is
performed and the name is re-bound to the result:
> a = a + 1
> a
array([ 2., 2., 2.])
> b
array([ 1., 1., 1.])
If you're writing code which could reasonably be expected to work with
arbitrary "numeric" values, you should decide which to use according to
whether in-place modification is appropriate rather than trivial
performance differences. If a difference of a few percent is significant,
Python is probably the wrong language in the first place.
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