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Newsgroups: comp.lang.python
Date: Thu, 15 Mar 2012 10:13:41 -0700 (PDT)
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Subject: Re: Style question (Poll)
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Xref: csiph.com comp.lang.python:21695

On Wednesday, 14 March 2012 21:16:05 UTC, Terry Reedy  wrote:
> 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

I'm not sure it's efficient or even if I like it, but it avoids try/except and the use of a for loop.

if next( iter(mylist), object() ) is not another_value:
    # ...

Just my 2p,

Jon.