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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Style question (Poll)
Date: Wed, 14 Mar 2012 17:16:05 -0400
References: <CANm61jcTZNVJAbkj2ZGJ+X1-mUZOoXsR90VstC4rb8UOo1x8Ug@mail.gmail.com> <CAJ6cK1b4Kop1uvEKLXUV3qBZkAajf+g-+i3pTksUVHxazy5EFg@mail.gmail.com>
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On 3/14/2012 4:49 PM, Arnaud Delobelle wrote:
> On 14 March 2012 20:37, Croepha<croepha@gmail.com>  wrote:
>> Which is preferred:
>>
>> for value in list:
>>   if not value is another_value:
>>     value.do_something()
>>     break

Do you really mean 'is' or '=='?

If you mean x is not y, write it that way.
'not x is y' can be misread and misunderstood, depending on whether
the 'is' is true or not.

 >>> not 1 is 1
False
 >>> not (1 is 1)
False
 >>> (not 1) is 1
False

Does not matter how read.

 >>> not (1 is 0)
True
 >>> (not 1) is 0
False
 >>> not 1 is 0
True

Does matter how read.

>> if list and not list[0] is another_value:
>>   list[0].do_something()

Or
try:
   value = mylist[0]
   if value is not another_value: value.dosomething
except IndexError:
   pass

I would not do this in this case of index 0, but if the index were a 
complicated expression or expensive function call, making 'if list' an 
inadequate test, I might.

> Hard to say, since they don't do the same thing :)
>
> I suspect you meant:
>
> for value in list:
>     if not value is another_value:
>         value.do_something()
>     break
>
> I always feel uncomfortable with this because it's misleading: a loop
> that never loops.

I agree. Please do not do this in public ;-).

-- 
Terry Jan Reedy