invert or reverse a string... warning this is a rant

[rick]

Why can't Python have a reverse() function/method like Ruby?

Python:
x = 'a_string'
# Reverse the string
print x[::-1]

Ruby:
x = 'a_string'
# Reverse the string
print x.reverse

The Ruby approach makes sense to me as a human being. The Python
approach is not easy for me (as a human being) to remember. Can that be
changed or should I just start blindly memorizing this stuff?

P.S. I like Python better than Ruby 90% of the time and use Python 90%
of the time, but 10% of the time, little things such as this drive me crazy!

[John Salerno]

Re: invert or reverse a string... warning this is a rant

rick wrote:

Why can't Python have a reverse() function/method like Ruby?

I'm not steeped enough in daily programming to argue that it isn't
necessary, but my question is why do you need to reverse strings? Is it
something that happens often enough to warrant a method for it?

[Paul Rubin]

Re: invert or reverse a string... warning this is a rant

rick <ath-admin@vt.eduwri tes:

Why can't Python have a reverse() function/method like Ruby?
>
Python:
x = 'a_string'
# Reverse the string
print x[::-1]
>
The Ruby approach makes sense to me as a human being. The Python
approach is not easy for me (as a human being) to remember. Can that
be changed or should I just start blindly memorizing this stuff?

You could use:

print ''.join(reverse d(x))

That also looks a little bit weird, but it combines well-known Python
idioms straightforward ly.

[Fredrik Lundh]

Re: invert or reverse a string... warning this is a rant

rick wrote:

The Ruby approach makes sense to me as a human being.

do the humans on your planet spend a lot of time reversing strings?
it's definitely not a very common thing to do over here.

anyway, if you do this a lot, why not define a helper function?

def reverse(s):
return s[::-1]

print reverse("redael ruoy ot em ekat")

</F>

[Brad]

Re: invert or reverse a string... warning this is a rant

John Salerno wrote:

rick wrote:

>Why can't Python have a reverse() function/method like Ruby?

>
I'm not steeped enough in daily programming to argue that it isn't
necessary, but my question is why do you need to reverse strings? Is it
something that happens often enough to warrant a method for it?

I'm home for lunch so my email addy is different.

No, it doesn't happen very often, but when I need to reverse something
(usually a list or a string). I can never remember right of the top of
my head how to do so in Python. I always have to Google for an answer or
refer back to old code.

IMO, I should be able to intuitively know how to do this. Python is so
user-friendly most every place else... why can it not be so here?

I wrote this so I'll never have to remember this again:

def invert(invertab le_object):
try:
print invertable_obje ct[::-1]
return invertable_obje ct[::-1]
except:
print 'Object not invertable'
return 1

invert([1,2,3,4])
invert('string' )
invert({1:2, 3:4})

[Neil Cerutti]

Re: invert or reverse a string... warning this is a rant

On 2006-10-19, Brad <rtilley@vt.edu wrote:

I'm home for lunch so my email addy is different.
>
No, it doesn't happen very often, but when I need to reverse
something (usually a list or a string). I can never remember
right of the top of my head how to do so in Python. I always
have to Google for an answer or refer back to old code.
>
IMO, I should be able to intuitively know how to do this.
Python is so user-friendly most every place else... why can it
not be so here?
>
I wrote this so I'll never have to remember this again:
>
def invert(invertab le_object):
try:
print invertable_obje ct[::-1]
return invertable_obje ct[::-1]
except:
print 'Object not invertable'
return 1
>
invert([1,2,3,4])
invert('string' )
invert({1:2, 3:4})

Shoot, now you'll have to remember where in heck you stashed that
function the next time you need to reverse something. ;-)

You'll still be better off in the long run memorizing the slice
notation.

--
Neil Cerutti

[Brad]

Re: invert or reverse a string... warning this is a rant

Fredrik Lundh wrote:

rick wrote:
>

>The Ruby approach makes sense to me as a human being.

>
do the humans on your planet spend a lot of time reversing strings? it's
definitely not a very common thing to do over here.

On our planet, we're all dyslexic. We tend to do things 'backwards' so
being able to easily invert what we do helps the people we show the code
to on your planet make sense of it.

>
anyway, if you do this a lot, why not define a helper function?
>
def reverse(s):
return s[::-1]
>
print reverse("redael ruoy ot em ekat")

Thanks, that's what I ended up doing.

[James Stroud]

Re: invert or reverse a string... warning this is a rant

John Salerno wrote:

rick wrote:
>

>Why can't Python have a reverse() function/method like Ruby?

>
>
I'm not steeped enough in daily programming to argue that it isn't
necessary, but my question is why do you need to reverse strings?

It would provide symmetry for reversing any sequence (without requiring
an iterator).

(1,2,3).reverse d()

"123".reversed( )

[1,2,3].reversed()



--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/

[Paul Boddie]

Re: invert or reverse a string... warning this is a rant

James Stroud wrote:

>
It would provide symmetry for reversing any sequence (without requiring
an iterator).
>
(1,2,3).reverse d()
>
"123".reversed( )
>
[1,2,3].reversed()

That might infuriate those who regard strings as "mischievou s"
sequences (ie. things which cause errors because you think you have a
list, but you're actually accessing characters in a string), but it's a
valid point: the built-in sequence types should, within reason, provide
a similar interface.

The proposed solution involving slicing syntax does seem odd in the
sense that it's highly similar to the redundant-for-strings s[:]
operation, and it might require inspiration for anyone looking for
convenience methods on the string object to consider using slicing
instead in this manner. In looking for alternative approaches, I became
somewhat distracted by the classic approach you'd use with certain
functional languages: convert the string into a list of characters (if
it isn't already treated as a list), reverse the list, concatenate the
list. The following requires Python 2.4:

"".join(list(re versed(list(s)) ))

I guess Python 2.5 has the reversed method of which you speak. Still,
the simplest things are often the best, and Python's classic operators
(indexing, slicing) should always be kept in mind.

Paul

[James Stroud]

Re: invert or reverse a string... warning this is a rant

Paul Boddie wrote:

James Stroud wrote:

>>(1,2,3).rever sed()
>>
>>"123".reverse d()
>>
>>[1,2,3].reversed()

>
I guess Python 2.5 has the reversed method of which you speak.

Not that I could find (as methods of any built in sequence type). 2.5
just has the "reversed" function that returns and iterator and so 2.5
requires these kind of gymnastics on built in sequences:

"".join(list(re versed(list(s)) ))

Of course, I think str.join can operate on iterators, as Paul Rubin
suggests:

print ''.join(reverse d(x))

This latter approach still seems a little clunky, though.

James


--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/

[Ron Adam]

Re: invert or reverse a string... warning this is a rant

James Stroud wrote:

Of course, I think str.join can operate on iterators, as Paul Rubin
suggests:
>

print ''.join(reverse d(x))

>
This latter approach still seems a little clunky, though.
>
James

Slices can be named so you could do...

>>reverser = slice(None, None, -1)
>>>
>>'abcdefg'[reverser]

'gfedcba'


Ron

[Fredrik Lundh]

Re: invert or reverse a string... warning this is a rant

James Stroud wrote:

without requiring an iterator

can we perhaps invent some more arbitrary constraints while we're at it?

</F>

[James Stroud]

Re: invert or reverse a string... warning this is a rant

Fredrik Lundh wrote:

James Stroud wrote:
>

without requiring an iterator

>
can we perhaps invent some more arbitrary constraints while we're at it?
>
</F>
>

Why does it seem to me that you are confusing convienience with
constraint, or are the two equivalent?

James

--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/

[Neil Cerutti]

Re: invert or reverse a string... warning this is a rant

On 2006-10-19, Fredrik Lundh <fredrik@python ware.comwrote:

James Stroud wrote:
>

without requiring an iterator

>
can we perhaps invent some more arbitrary constraints while
we're at it?

No letter G. I don't like them. They wet their nests.

--
Neil Cerutti
You only get a once-in-a-lifetime opportunity so many times.
--Ike Taylor

[James Stroud]

Re: invert or reverse a string... warning this is a rant

Fredrik Lundh wrote:

James Stroud wrote:
>

without requiring an iterator

>
can we perhaps invent some more arbitrary constraints while we're at it?
>
</F>
>

I guess while I'm at it, this thread wouldn't have so much steam were
these idioms seemingly unpythonic:

"".join(reverse (x))
alist[::-1]

The latter, while more terse than alist.reversed( ), is unnatural and
ugly compared to the general elegance of other constructs in python.
Were this not the case, beginners and intermediate programmers alike
would not have such trouble remembering it. In fact, the latter's exact
behavior, if I remember correctly, spawned a thread of its own with much
speculation as to the exact meaning of the documentation and whether the
API conformed to this inferred meaning. I'll attempt to provide a link
to the thread if anyone takes me to task on this.

I'm sure many life-long programmers will claim that they have never
created a reverse copy of a data structure for production code, but why
is it that so many jump to the fore to point out that alist[::-1] is how
one produces a reverse copy of a list, or that a string can be reversed
with reversed, join, and an instance of another string (did I leave
something out?)?

But why do so many beginning programmers ask how one might produce a
reverse data structure in python? Perhaps they are just ignorant fools
who don't know that creating a reverse copy of a data structure can be
proven to be a useless operation if only they would stop trying to write
code and begin to write formal proofs. Perhaps their professors are to
blame, unreasonably asking them to write actual code before memorizing
all three+ volumes of Knuth. I'm sure the proof is in there somewhere.

But maybe it is not the purpose of a poweful language like python to be
used as a teaching language. Maybe python should be reserved for use
only by those who have been desensitized to its idiosyncracies,
inconsistencies , and idiomatic workarounds.

James

--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/