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A curious bit of code...

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  A curious bit of code... forman.simon@gmail.com - 2014-02-13 10:37 -0800
    Re: A curious bit of code... Roy Smith <roy@panix.com> - 2014-02-13 13:45 -0500
    Re: A curious bit of code... Ethan Furman <ethan@stoneleaf.us> - 2014-02-13 10:45 -0800
    Re: A curious bit of code... Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-02-13 19:09 +0000
    Re: A curious bit of code... Alain Ketterlin <alain@dpt-info.u-strasbg.fr> - 2014-02-13 20:05 +0100
    Re: A curious bit of code... Neil Cerutti <neilc@norwich.edu> - 2014-02-13 19:17 +0000
    Re: A curious bit of code... Ethan Furman <ethan@stoneleaf.us> - 2014-02-13 11:20 -0800
      Re: A curious bit of code... Roy Smith <roy@panix.com> - 2014-02-13 14:28 -0500
    Re: A curious bit of code... Ethan Furman <ethan@stoneleaf.us> - 2014-02-13 11:25 -0800
    Re: A curious bit of code... Neil Cerutti <neilc@norwich.edu> - 2014-02-13 19:25 +0000
    Re: A curious bit of code... Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-02-13 19:32 +0000
    Re: A curious bit of code... Peter Otten <__peter__@web.de> - 2014-02-13 20:43 +0100
      Re: A curious bit of code... Marko Rauhamaa <marko@pacujo.net> - 2014-02-13 21:56 +0200
    Re: A curious bit of code... Ethan Furman <ethan@stoneleaf.us> - 2014-02-13 11:23 -0800
    Re: A curious bit of code... Neil Cerutti <neilc@norwich.edu> - 2014-02-13 19:51 +0000
    Re: A curious bit of code... Ethan Furman <ethan@stoneleaf.us> - 2014-02-13 11:59 -0800
    Re: A curious bit of code... Zachary Ware <zachary.ware+pylist@gmail.com> - 2014-02-13 13:59 -0600
    Re: A curious bit of code... Chris Angelico <rosuav@gmail.com> - 2014-02-14 07:29 +1100
    Re: A curious bit of code... Tim Chase <python.list@tim.thechases.com> - 2014-02-13 14:39 -0600
    Re: A curious bit of code... Emile van Sebille <emile@fenx.com> - 2014-02-13 12:55 -0800
      Re: A curious bit of code... Roy Smith <roy@panix.com> - 2014-02-13 16:24 -0500
        Re: A curious bit of code... Ethan Furman <ethan@stoneleaf.us> - 2014-02-13 16:23 -0800
    Re: A curious bit of code... Chris Angelico <rosuav@gmail.com> - 2014-02-14 08:01 +1100
    Re: A curious bit of code... Neil Cerutti <neilc@norwich.edu> - 2014-02-13 21:01 +0000
    Re: A curious bit of code... Peter Otten <__peter__@web.de> - 2014-02-13 22:06 +0100
    Re: A curious bit of code... Chris Angelico <rosuav@gmail.com> - 2014-02-14 08:10 +1100
    Re: A curious bit of code... Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-02-13 21:14 +0000
    Re: A curious bit of code... Zachary Ware <zachary.ware+pylist@gmail.com> - 2014-02-13 15:20 -0600
    Re: A curious bit of code... Zachary Ware <zachary.ware+pylist@gmail.com> - 2014-02-13 15:19 -0600
    Re: A curious bit of code... Emile van Sebille <emile@fenx.com> - 2014-02-13 13:23 -0800
    Re: A curious bit of code... Chris Angelico <rosuav@gmail.com> - 2014-02-14 08:31 +1100
      Re: A curious bit of code... Roy Smith <roy@panix.com> - 2014-02-13 16:38 -0500
    Re: A curious bit of code... Zachary Ware <zachary.ware+pylist@gmail.com> - 2014-02-13 15:47 -0600
    Re: A curious bit of code... Serhiy Storchaka <storchaka@gmail.com> - 2014-02-13 23:49 +0200
      Re: A curious bit of code... Roy Smith <roy@panix.com> - 2014-02-13 16:51 -0500
    Re: A curious bit of code... Ethan Furman <ethan@stoneleaf.us> - 2014-02-13 13:33 -0800
    Re: A curious bit of code... Chris Angelico <rosuav@gmail.com> - 2014-02-14 09:13 +1100
    Re: A curious bit of code... Ethan Furman <ethan@stoneleaf.us> - 2014-02-13 14:26 -0800
    Re: A curious bit of code... Terry Reedy <tjreedy@udel.edu> - 2014-02-13 19:29 -0500
    Re: A curious bit of code... forman.simon@gmail.com - 2014-02-13 18:45 -0800
      Re: A curious bit of code... Ned Batchelder <ned@nedbatchelder.com> - 2014-02-13 22:26 -0500
        Re: A curious bit of code... forman.simon@gmail.com - 2014-02-14 12:04 -0800
          Re: A curious bit of code... Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-02-14 21:01 +0000
    Re: A curious bit of code... Dave Angel <davea@davea.name> - 2014-02-14 07:19 -0500
      Re: A curious bit of code... Roy Smith <roy@panix.com> - 2014-02-14 09:06 -0500

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#66249

In article <mailman.6880.1392324956.18130.python-list@python.org>,
 Emile van Sebille <emile@fenx.com> wrote:

> On 2/13/2014 11:59 AM, Zachary Ware wrote:
> > In a fit of curiosity, I did some timings:
> 
> Snip of lots of TMTOWTDT/TIMTOWTDI/whatever... timed examples :)
> 
> But I didn't see this one:
> 
> s[::len(s)-1]
> 
> Emile

I love it.  I need to add this to my list of Python trivia questions.

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#66273

On 02/13/2014 01:24 PM, Roy Smith wrote:
>   Emile van Sebille wrote:
>>
>> But I didn't see this one:
>>
>> s[::len(s)-1]
>
> I love it.  I need to add this to my list of Python trivia questions.

Great interview question:  What does this do?  What is its weakness?  How would you fix it?

--
~Ethan~

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#66240

On Fri, Feb 14, 2014 at 7:55 AM, Emile van Sebille <emile@fenx.com> wrote:
> On 2/13/2014 11:59 AM, Zachary Ware wrote:
>>
>> In a fit of curiosity, I did some timings:
>
>
> Snip of lots of TMTOWTDT/TIMTOWTDI/whatever... timed examples :)
>
> But I didn't see this one:
>
> s[::len(s)-1]

.... AAAAAAAAAAAAAAAAAAARGGGGGGGGGGHHHH!

Really, I actually did pause, double-take, and then scream under my
breath, when I saw that. Yes, it works. But please, if you EVER do
this, save me the trouble and just submit your code to thedailywtf.com
straight away!

ChrisA

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#66241

On 2014-02-13, Zachary Ware <zachary.ware+pylist@gmail.com> wrote:
> In a fit of curiosity, I did some timings:
>
> 'and'ed indexing:
>
> C:\tmp>py -m timeit -s "key = '<test>'" "key[0] == '<' and key[-1] == '>'"
> 1000000 loops, best of 3: 0.35 usec per loop
>
> C:\tmp>py -m timeit -s "key = '<test'" "key[0] == '<' and key[-1] == '>'"
> 1000000 loops, best of 3: 0.398 usec per loop
>
> C:\tmp>py -m timeit -s "key = 'test>'" "key[0] == '<' and key[-1] == '>'"
> 1000000 loops, best of 3: 0.188 usec per loop
>
> C:\tmp>py -m timeit -s "key = 'test'" "key[0] == '<' and key[-1] == '>'"
> 10000000 loops, best of 3: 0.211 usec per loop
>
> C:\tmp>py -m timeit -s "key = ''" "key[0] == '<' and key[-1] == '>'"
> Traceback (most recent call last):
>   File "P:\Python34\lib\timeit.py", line 292, in main
>     x = t.timeit(number)
>   File "P:\Python34\lib\timeit.py", line 178, in timeit
>     timing = self.inner(it, self.timer)
>   File "<timeit-src>", line 6, in inner
>     key[0] == '<' and key[-1] == '>'
> IndexError: string index out of range

The corrected version

key and key[0] == '<' and key[-1] == '>'

probably still wins the Pretty Unimportant Olympics.

-- 
Neil Cerutti

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#66242

Chris Angelico wrote:

> On Fri, Feb 14, 2014 at 7:55 AM, Emile van Sebille <emile@fenx.com> wrote:
>> On 2/13/2014 11:59 AM, Zachary Ware wrote:
>>>
>>> In a fit of curiosity, I did some timings:
>>
>>
>> Snip of lots of TMTOWTDT/TIMTOWTDI/whatever... timed examples :)
>>
>> But I didn't see this one:
>>
>> s[::len(s)-1]
> 
> .... AAAAAAAAAAAAAAAAAAARGGGGGGGGGGHHHH!
> 
> Really, I actually did pause, double-take, and then scream under my
> breath, when I saw that. Yes, it works. But please, if you EVER do
> this, save me the trouble and just submit your code to thedailywtf.com
> straight away!

For the record:

>>> s = "x"
>>> s[::len(s)-1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: slice step cannot be zero

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#66243

On Fri, Feb 14, 2014 at 8:06 AM, Peter Otten <__peter__@web.de> wrote:
> For the record:
>
>>>> s = "x"
>>>> s[::len(s)-1]
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> ValueError: slice step cannot be zero

And that, my friends, is a classic example of a Python exception that
ought to be a subclass of UnhingedProgrammerError.

ChrisA

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#66245

On 13/02/2014 21:01, Neil Cerutti wrote:
> On 2014-02-13, Zachary Ware <zachary.ware+pylist@gmail.com> wrote:
>> In a fit of curiosity, I did some timings:
>>
>> 'and'ed indexing:
>>
>> C:\tmp>py -m timeit -s "key = '<test>'" "key[0] == '<' and key[-1] == '>'"
>> 1000000 loops, best of 3: 0.35 usec per loop
>>
>> C:\tmp>py -m timeit -s "key = '<test'" "key[0] == '<' and key[-1] == '>'"
>> 1000000 loops, best of 3: 0.398 usec per loop
>>
>> C:\tmp>py -m timeit -s "key = 'test>'" "key[0] == '<' and key[-1] == '>'"
>> 1000000 loops, best of 3: 0.188 usec per loop
>>
>> C:\tmp>py -m timeit -s "key = 'test'" "key[0] == '<' and key[-1] == '>'"
>> 10000000 loops, best of 3: 0.211 usec per loop
>>
>> C:\tmp>py -m timeit -s "key = ''" "key[0] == '<' and key[-1] == '>'"
>> Traceback (most recent call last):
>>    File "P:\Python34\lib\timeit.py", line 292, in main
>>      x = t.timeit(number)
>>    File "P:\Python34\lib\timeit.py", line 178, in timeit
>>      timing = self.inner(it, self.timer)
>>    File "<timeit-src>", line 6, in inner
>>      key[0] == '<' and key[-1] == '>'
>> IndexError: string index out of range
>
> The corrected version
>
> key and key[0] == '<' and key[-1] == '>'
>
> probably still wins the Pretty Unimportant Olympics.
>

Exactly how I'd write it.  To me it wins awards for being most boring 
and most obvious, obviously YMMV or we wouldn't be having this 
discussion.  Or argument.  Or contradiction :)

-- 
My fellow Pythonistas, ask not what our language can do for you, ask 
what you can do for our language.

Mark Lawrence

---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

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#66246

On Thu, Feb 13, 2014 at 3:01 PM, Neil Cerutti <neilc@norwich.edu> wrote:
> On 2014-02-13, Zachary Ware <zachary.ware+pylist@gmail.com> wrote:
>> C:\tmp>py -m timeit -s "key = ''" "key[0] == '<' and key[-1] == '>'"
>> Traceback (most recent call last):
>>   File "P:\Python34\lib\timeit.py", line 292, in main
>>     x = t.timeit(number)
>>   File "P:\Python34\lib\timeit.py", line 178, in timeit
>>     timing = self.inner(it, self.timer)
>>   File "<timeit-src>", line 6, in inner
>>     key[0] == '<' and key[-1] == '>'
>> IndexError: string index out of range
>
> The corrected version
>
> key and key[0] == '<' and key[-1] == '>'
>
> probably still wins the Pretty Unimportant Olympics.

Indeed, see the last set of timings and moral ;)

-- 
Zach

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#66247

On Thu, Feb 13, 2014 at 2:55 PM, Emile van Sebille <emile@fenx.com> wrote:
> On 2/13/2014 11:59 AM, Zachary Ware wrote:
>>
>> In a fit of curiosity, I did some timings:
>
>
> Snip of lots of TMTOWTDT/TIMTOWTDI/whatever... timed examples :)
>
> But I didn't see this one:
>
> s[::len(s)-1]

It's not great, around 0.520 usec (the builtin lookup and function
call is what gets it).  Also, uglier than sin itself.

-- 
Zach

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#66248

On 2/13/2014 1:10 PM, Chris Angelico wrote:
> On Fri, Feb 14, 2014 at 8:06 AM, Peter Otten <__peter__@web.de> wrote:
>> For the record:
>>
>>>>> s = "x"
>>>>> s[::len(s)-1]
>> Traceback (most recent call last):
>>    File "<stdin>", line 1, in <module>
>> ValueError: slice step cannot be zero
>
> And that, my friends, is a classic example of a Python exception that
> ought to be a subclass of UnhingedProgrammerError.
>

And certainly s[::len(s)-1 or 1] isn't any better.  :)

Emile

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#66254

On Fri, Feb 14, 2014 at 8:19 AM, Zachary Ware
<zachary.ware+pylist@gmail.com> wrote:
> Also, uglier than sin itself.

Hey hey, no need to insult our lovely trigonometric functions!

ChrisA

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#66255

In article <mailman.6891.1392327074.18130.python-list@python.org>,
 Chris Angelico <rosuav@gmail.com> wrote:

> On Fri, Feb 14, 2014 at 8:19 AM, Zachary Ware
> <zachary.ware+pylist@gmail.com> wrote:
> > Also, uglier than sin itself.
> 
> Hey hey, no need to insult our lovely trigonometric functions!

This newsgroup is taylor made for that kind of abuse.

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#66256

On Thu, Feb 13, 2014 at 3:31 PM, Chris Angelico <rosuav@gmail.com> wrote:
> On Fri, Feb 14, 2014 at 8:19 AM, Zachary Ware
> <zachary.ware+pylist@gmail.com> wrote:
>> Also, uglier than sin itself.
>
> Hey hey, no need to insult our lovely trigonometric functions!

Here's your sine...

-- 
Zach

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#66257

13.02.14 21:59, Zachary Ware написав(ла):
> don't use re for simple stuff (because while it may be very fast, it's
> dominated by attribute lookup and function call overhead),

And the time of re variant depends on the size of input.

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#66258

In article <mailman.6893.1392328170.18130.python-list@python.org>,
 Serhiy Storchaka <storchaka@gmail.com> wrote:

> 13.02.14 21:59, Zachary Ware написав(ла):
> > don't use re for simple stuff (because while it may be very fast, it's
> > dominated by attribute lookup and function call overhead),
> 
> And the time of re variant depends on the size of input.

That's a good point.  It's nice, for a change, to see somebody shoot 
down a regex solution for a valid reason :-)

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#66259

On 02/13/2014 01:01 PM, Chris Angelico wrote:
> On Fri, Feb 14, 2014 at 7:55 AM, Emile van Sebille <emile@fenx.com> wrote:
>> On 2/13/2014 11:59 AM, Zachary Ware wrote:
>>>
>>> In a fit of curiosity, I did some timings:
>>
>>
>> Snip of lots of TMTOWTDT/TIMTOWTDI/whatever... timed examples :)
>>
>> But I didn't see this one:
>>
>> s[::len(s)-1]
>
> .... AAAAAAAAAAAAAAAAAAARGGGGGGGGGGHHHH!
>
> Really, I actually did pause, double-take, and then scream under my
> breath, when I saw that. Yes, it works. But please, if you EVER do
> this, save me the trouble and just submit your code to thedailywtf.com
> straight away!

Oh, it's not that bad!  All you have to do is handle the edge case of an empty string:

s[::len(s)-1 if s else True]

*ducks and runs*

--
~Ethan~

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#66262

On Fri, Feb 14, 2014 at 8:33 AM, Ethan Furman <ethan@stoneleaf.us> wrote:
> Oh, it's not that bad!  All you have to do is handle the edge case of an
> empty string:
>
> s[::len(s)-1 if s else True]
>
> *ducks and runs*

And the edge case of the one-character string. Also, it's been noted
that calling the built-in function len() is slow, so I propose
changing that.

s[::(sys.getsizeof("\N{MATHEMATICAL BOLD CAPITAL
L}"+s)-sys.getsizeof("\N{MATHEMATICAL BOLD CAPITAL E}\N{MATHEMATICAL
BOLD CAPITAL N}") or 4)//4]

Note the elegance of using the word LEN across two string literals
(because string literals are fast) as a means of clearly describing
what we are doing - an optimization of the built-in len() function.

ChrisA

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#66266

On 02/13/2014 02:13 PM, Chris Angelico wrote:
> On Fri, Feb 14, 2014 at 8:33 AM, Ethan Furman <ethan@stoneleaf.us> wrote:
>> Oh, it's not that bad!  All you have to do is handle the edge case of an
>> empty string:
>>
>> s[::len(s)-1 if s else True]
>
> And the edge case of the one-character string.

Oops, my description should have said "edge case of a one-character string".  The empty string needs no extra handling.

--
~Ethan~

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#66272

On 2/13/2014 1:37 PM, forman.simon@gmail.com wrote:
> I ran across this and I thought there must be a better way of doing it, but then after further consideration I wasn't so sure.
>
>    if key[:1] + key[-1:] == '<>': ...

if key[:1] == '<' and key[-1:] == '>: ...
is the obvious choice to me. If the first clause is false, it never 
computes the second.

-- 
Terry Jan Reedy

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#66275

For the record I wasn't worried about the performance.  ;-)

It was for Tkinter event strings not markup tags.

I'm glad this was the time winner!

"key and key[0] == '<' and key[-1] == '>'"


Cheers to the folks who did the timings (and saved me from the trouble!)

Last but not least...  s[::len(s)-1]   omg!!?   ;-D

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