String concatenation

[Leif B. Kristensen]

Having recently started with Python, I've written this little function
to retrieve place parts from a database and concatenate them to a
string. While it certainly works, and is also considerably shorter than
the PHP code that I originally wrote, I'm pretty convinced that there
should be an even better way to do it. Can anybody show me how to write
the string concatenation part in a more Pythonesque syntax?

def get_place(x):
c=db.cursor()
c.execute("sele ct place_lvl1, place_lvl2, \
place_lvl3, place_lvl4, place_lvl5 \
from place where place_id = %d" % (x))
res=c.fetchone( )
place = ''
for i in range(5):
tmp = res[i]
if tmp[:1] != '-' and len(tmp) != 0:
place = place + ', ' + (res[i])
return place[2:]

regards,
--
Leif Biberg Kristensen

http://solumslekt.org/

Validare necesse est

[Paul Rubin]

Re: String concatenation

"Leif B. Kristensen" <junkmail@solum slekt.org> writes:[color=blue]
> Having recently started with Python, I've written this little function
> to retrieve place parts from a database and concatenate them to a
> string. While it certainly works, and is also considerably shorter than
> the PHP code that I originally wrote, I'm pretty convinced that there
> should be an even better way to do it. Can anybody show me how to write
> the string concatenation part in a more Pythonesque syntax?
> ...
> for i in range(5):
> tmp = res[i]
> if tmp[:1] != '-' and len(tmp) != 0:
> place = place + ', ' + (res[i])
> return place[2:][/color]

Not tested:

tmp = [x for x in res if x and x[0] != '-']
return ', '.join(tmp)

Explanation:

1) you can use "for x in res" instead of looping through range(5),
since you know that res has 5 elements (from the sql result).

2) x[:1] is the same as x[0] as long as x has at least 1 char (otherwise
it throws an exception)

3) To prevent the exception, test for x being nonempty BEFORE examining x[0]

4) sep.join(string list) is the standard way to join a bunch of strings
together, separated by sep.

[Leif B. Kristensen]

Re: String concatenation

Paul Rubin wrote:
[color=blue]
> Not tested:
>
> tmp = [x for x in res if x and x[0] != '-']
> return ', '.join(tmp)[/color]

Paul,
that looks a lot more like the examples in "Dive Into Python" which I'm
reading at the moment. But the snake won't swallow:

tmp = [x for x in res if x and x[0] != '-']
^
SyntaxError: invalid syntax

I wonder what it's really trying to tell me?

regards,
--
Leif Biberg Kristensen

http://solumslekt.org/

Validare necesse est

[Erik Max Francis]

Re: String concatenation

"Leif B. Kristensen" wrote:
[color=blue]
> that looks a lot more like the examples in "Dive Into Python" which
> I'm
> reading at the moment. But the snake won't swallow:
>
> tmp = [x for x in res if x and x[0] != '-']
> ^
> SyntaxError: invalid syntax
>
> I wonder what it's really trying to tell me?[/color]

Probably you included the initial spaces, which is a no-no:
[color=blue][color=green][color=darkred]
>>> a = 1
>>> b = 2[/color][/color][/color]
File "<stdin>", line 1
b = 2
^
SyntaxError: invalid syntax

--
__ Erik Max Francis && max@alcyone.com && http://www.alcyone.com/max/
/ \ San Jose, CA, USA && 37 20 N 121 53 W && AIM erikmaxfrancis
\__/ They love too much that die for love.
-- (an English proverb)

[Leif B. Kristensen]

Re: String concatenation

Erik Max Francis wrote:
[color=blue]
> Probably you included the initial spaces, which is a no-no:[/color]

No I didn't. The text is properly aligned. The relevant code now looks
exactly like this:

res=c.fetchone( )
tmp=[x for x in res if x and x[0] != '-']
return ', '.join(tmp)

And here's what the interpreter says:

tmp=[x for x in res if x and x[0] != '-']
^
SyntaxError: invalid syntax

The marker has moved to the right, maybe because I removed the spaces on
both sides of the = sign?

regards,
--
Leif Biberg Kristensen

http://solumslekt.org/

Validare necesse est

[Erik Max Francis]

Re: String concatenation

"Leif B. Kristensen" wrote:
[color=blue]
> And here's what the interpreter says:
>
> tmp=[x for x in res if x and x[0] != '-']
> ^
> SyntaxError: invalid syntax
>
> The marker has moved to the right, maybe because I removed the spaces
> on
> both sides of the = sign?[/color]

At this point I don't know what's going on since there's a communication
problem; Python's exception specification is based on the code you
entered, not the code it thought you meant, so you're having a
cutting-and-pasting problem at least on one side here.

If indentation is not to blame, it's possible that the problem is that
you're not running a version of Python that understands list
comprehensions.

--
__ Erik Max Francis && max@alcyone.com && http://www.alcyone.com/max/
/ \ San Jose, CA, USA && 37 20 N 121 53 W && AIM erikmaxfrancis
\__/ They love too much that die for love.
-- (an English proverb)

[Leif B. Kristensen]

Re: String concatenation

Erik Max Francis wrote:
[color=blue]
> At this point I don't know what's going on since there's a
> communication problem; Python's exception specification is based on
> the code you entered, not the code it thought you meant, so you're
> having a cutting-and-pasting problem at least on one side here.[/color]

Erik,
After entering Paul's two lines by hand, it suddenly works. You must be
right about the cutting-and-pasting problem, though I don't understand
why lifting text from KNode to KWrite shouldn't work.
[color=blue]
> If indentation is not to blame, it's possible that the problem is that
> you're not running a version of Python that understands list
> comprehensions.[/color]

bash-2.05b$ python
Python 2.3.3 (#1, Jan 24 2004, 10:16:14)
[GCC 3.2.3 20030422 (Gentoo Linux 1.4 3.2.3-r3, propolice)] on linux2
Type "help", "copyright" , "credits" or "license" for more information.

regards,
--
Leif Biberg Kristensen

http://solumslekt.org/

Validare necesse est

[Erik Max Francis]

Re: String concatenation

"Leif B. Kristensen" wrote:
[color=blue]
> After entering Paul's two lines by hand, it suddenly works. You must
> be
> right about the cutting-and-pasting problem, though I don't understand
> why lifting text from KNode to KWrite shouldn't work.[/color]

Maybe it's grabbing tabs, or obscure unprintable whitespace you can't
see. Certainly what you're pasting in to these posts is intended,
whether or not you did that manually.

--
__ Erik Max Francis && max@alcyone.com && http://www.alcyone.com/max/
/ \ San Jose, CA, USA && 37 20 N 121 53 W && AIM erikmaxfrancis
\__/ It's soulful music. It doesn't necessarily sound like ... soul ...
-- Sade Adu

[Leif B. Kristensen]

Re: String concatenation

Paul Rubin wrote:
[color=blue]
> "Leif B. Kristensen" <junkmail@solum slekt.org> writes:[color=green]
>> for i in range(5):
>> tmp = res[i]
>> if tmp[:1] != '-' and len(tmp) != 0:
>> place = place + ', ' + (res[i])
>> return place[2:][/color]
>
> tmp = [x for x in res if x and x[0] != '-']
> return ', '.join(tmp)[/color]

Now I got it working, and it's really neat! It's almost incredible that
you can do all this with just two lines of code.

Thank you very much.

regards,
--
Leif Biberg Kristensen

http://solumslekt.org/

Validare necesse est

[Leif B. Kristensen]

Re: String concatenation

Erik Max Francis wrote:
[color=blue]
> Maybe it's grabbing tabs, or obscure unprintable whitespace you can't
> see. Certainly what you're pasting in to these posts is intended,
> whether or not you did that manually.[/color]

It may be a bug in KWrite. As I entered it manually, I noticed that the
pasted text wasn't syntax highlighted.

Yeah, I know -- I should use a real editor, like vi or emacs ;-)

regards,
--
Leif Biberg Kristensen

http://solumslekt.org/

Validare necesse est

[Leif B. Kristensen]

Re: String concatenation

Erik Max Francis wrote:
[color=blue]
> Maybe it's grabbing tabs, or obscure unprintable whitespace you can't
> see. Certainly what you're pasting in to these posts is intended,
> whether or not you did that manually.[/color]

The problem is recognised and confirmed. I posted a description of the
situation on comp.windows.x. kde, and received essentially this answer:

"you will notice that the "space" is an ASCII space (20 hex) in the
working version and ASCII space + 128 (A0 hex) in the non-working
version. Just because the display looks the same does not mean it is
the same."

regards,
--
Leif Biberg Kristensen

http://solumslekt.org/

Validare necesse est

[Leif B. Kristensen]

Re: String concatenation

The "feature" is also submitted to the KDE bugtrack, see
<url:http://bugs.kde.org/show_bug.cgi?id =60971>.

Apparently, "real programmers" don't cut and paste code samples from
KNode. Go figure.

regards,
--
Leif Biberg Kristensen

http://solumslekt.org/

Validare necesse est

[Peter Otten]

Re: String concatenation

Leif B. Kristensen wrote:
[color=blue]
> The "feature" is also submitted to the KDE bugtrack, see
> <url:http://bugs.kde.org/show_bug.cgi?id =60971>.
>
> Apparently, "real programmers" don't cut and paste code samples from
> KNode. Go figure.[/color]

I doubt that KNode is to blame. The problem you encountered occured
to me only sporadically. Therefore I guess that the culprit is elsewhere in
the toolchain.

In the meantime, whenever an apparent whitespace generates a syntax error:

<clean160.py>
#!/usr/bin/env python
import string, sys, os

transTable = string.maketran s(chr(160), " ")

if __name__ == "__main__":
for filename in sys.argv[1:]:
s = file(filename). read()
if chr(160) in s:
print "cleaning", filename
os.rename(filen ame, filename + ".160")
file(filename, "w").write(s.tr anslate(transTa ble))
else:
print "skipping", filename
<clean160.py>

Peter

PS: I'm not sure if the above reaches you without those vicious blanks, but
I'm confident :-)

[Leif B. Kristensen]

Re: String concatenation

Paul Rubin wrote:
[color=blue]
> tmp = [x for x in res if x and x[0] != '-']
> return ', '.join(tmp)[/color]

Substituting tmp with the real thing, it turns into a one-liner:

return ', '.join([x for x in res if x and x[0] != '-'])

Awesome :-)
--
Leif Biberg Kristensen

http://solumslekt.org/

Validare necesse est

[Paul Rubin]

Re: String concatenation

"Leif B. Kristensen" <junkmail@solum slekt.org> writes:[color=blue][color=green]
> > tmp = [x for x in res if x and x[0] != '-']
> > return ', '.join(tmp)[/color]
>
> Substituting tmp with the real thing, it turns into a one-liner:
>
> return ', '.join([x for x in res if x and x[0] != '-'])
>
> Awesome :-)[/color]

Yeah, I wrote it that way at first, but figured it was obscure enough
already if you weren't used to list comprehensions, so I put back the
tmp variable.