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<croe...@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.
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