Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!aioe.org!feeder.news-service.com!xlned.com!feeder5.xlned.com!newsfeed.xs4all.nl!newsfeed5.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: relative speed of incremention syntaxes (or "i=i+1" VS "i+=1")
Date: Sun, 21 Aug 2011 20:35:32 -0400
References: <8c606fc1-0aa8-4113-b607-e46ad6f3d649@glegroupsg2000goo.googlegroups.com> <pan.2011.08.21.21.07.55.886000@nowhere.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:6.0) Gecko/20110812 Thunderbird/6.0
In-Reply-To: <pan.2011.08.21.21.07.55.886000@nowhere.com>
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.294.1313974278.27778.python-list@python.org>
Lines: 84
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:11978

On 8/21/2011 5:07 PM, Nobody wrote:

> If the value on the left has an __iadd__ method, that will be called;

Correct

> the value will be updated in-place,

Not necessarily correct. The target is rebound to the return from the 
__iadd__ method. Augmented *assignment* is always assignment. This trips 
up people who try

t = (1, [])
t[1] += [1,2] # which *does* extend the array, but raises
TypeError: 'tuple' object does not support item assignment
# instead of
t[1].extend([1,2]) # which extends without raising an error


*IF* (and only if) the target object is mutable, then the __iadd__ may 
optionally mutate the target object. But the rebinding takes place 
nonetheless. Numbers, the subject of this thread, are not mutable and 
are not 'updated in-place'

class test:
         def __init__(self, n):
                 self.n = n
         def __iadd__(self, other):
                 return test(self.n + other.n)
         def __repr__(self):
                 return repr(self.n)

r = test(1)
t = r
t += r
print(r, t)
# 1,2

That said, there is normally no reason to write an __ixxx__ method 
unless instances are mutable and the operation can be and is done in 
place therein. So the class above is for illustrative purposes only. A 
saner example is the following, which treats test examples as mutable 
number containers rather than as immutable surrogates.

class test:
         def __init__(self, n):
                 self.n = n
         def __add__(self, other):
                 return test(self.n + other.n)
         def __iadd__(self, other):
                 n = self.n + other.n
                 self.n = n
                 return n
         def __repr__(self):
                 return repr(self.n)

r = test(1)
t = r
t += r
print(r, t)
# 2 2

The interpreter cannot enforce that 'x += a' have the same effect as 'x 
= x+a', but it would break normal expectations to make the two different.


 > so all references to that object will be affected:

Only if the target object is mutable and is mutated by the optional 
augmented assignment __ixxx__ methods.

> 	>  import numpy as np
> 	>  a = np.zeros(3)

Numpy arrays meet the qualification above.

> 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:

As is also done with the results of __ixxx__ methods.

-- 
Terry Jan Reedy