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Groups > comp.lang.python > #21626 > unrolled thread
| Started by | Terry Reedy <tjreedy@udel.edu> |
|---|---|
| First post | 2012-03-14 17:16 -0400 |
| Last post | 2012-03-15 10:13 -0700 |
| Articles | 3 — 2 participants |
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Re: Style question (Poll) Terry Reedy <tjreedy@udel.edu> - 2012-03-14 17:16 -0400
Re: Style question (Poll) Jon Clements <joncle@googlemail.com> - 2012-03-15 10:13 -0700
Re: Style question (Poll) Jon Clements <joncle@googlemail.com> - 2012-03-15 10:13 -0700
| From | Terry Reedy <tjreedy@udel.edu> |
|---|---|
| Date | 2012-03-14 17:16 -0400 |
| Subject | Re: Style question (Poll) |
| Message-ID | <mailman.653.1331759783.3037.python-list@python.org> |
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
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| From | Jon Clements <joncle@googlemail.com> |
|---|---|
| Date | 2012-03-15 10:13 -0700 |
| Message-ID | <24625945.5378.1331831621256.JavaMail.geo-discussion-forums@vblb5> |
| In reply to | #21626 |
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.
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| From | Jon Clements <joncle@googlemail.com> |
|---|---|
| Date | 2012-03-15 10:13 -0700 |
| Message-ID | <mailman.688.1331831625.3037.python-list@python.org> |
| In reply to | #21626 |
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.
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