A curious bit of code...

[Neil Cerutti]

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.

 

[Chris Angelico]

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

 

[Peter Otten]

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

 

[Chris Angelico]

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

 

[Mark Lawrence]

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

 

[Zachary Ware]

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.

 

[Zachary Ware]

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

 

[Emile van Sebille]

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

 

[Roy Smith]

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.

 

[Chris Angelico]

Also, uglier than sin itself.

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

ChrisA

 

[Ethan Furman]

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*

 

[Roy Smith]

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

This newsgroup is taylor made for that kind of abuse.

 

[Zachary Ware]

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

Here's your sine...

 

[Serhiy Storchaka]

13.02.14 21:59, Zachary Ware напиÑав(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.

 

[Roy Smith]

13.02.14 21:59, Zachary Ware напиÑав(ла):

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

 

[Chris Angelico]

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

 

[Ethan Furman]

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 Furman]

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?

 

[Terry Reedy]

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.

 

[forman.simon]

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