raw strings and \

[plahey]

I thought I understood raw strings, then I got bitten by this:

x=r'c:\blah\'

which is illegal! I thought that \ had no special meanning in a raw
string so why can't it be the last character?

making me do:

x=r'c:\blah' '\\'

is just silly...

[Duncan Booth]

wrote:

I thought I understood raw strings, then I got bitten by this:

x=r'c:\blah\'

which is illegal! I thought that \ had no special meanning in a raw
string so why can't it be the last character?
No, the backslash is still special in terms of parsing the string, it
is simply that once the string has been parsed escape sequences are not
interpreted.

making me do:

x=r'c:\blah' '\\'

is just silly...

Yes, that is silly. You could alternatively do:

x = 'c:/blah/'

or

x = 'c:\\blah\\'

But why would you want to end your path with a literal backslash anyway?
Get into the habit of using the os.path module for all manipulations of
file paths and you never need to worry about this sort of thing again.

This sort of thing should work well enough:

folder = 'c:/blah'

for line in open(os.path.jo in(folder, 'data.txt')):
print line

[plahey]

Hi Duncan,

thanks for the reply. I figured that this was a technical problem
associated with the parser.

This one is going on my Python gotchas list. It is really silly from
an end user perspective to have \ not special in raw strings _except_
if it is the last character.

[Alex Martelli]

<pl****@alumni. caltech.edu> wrote:

Hi Duncan,

thanks for the reply. I figured that this was a technical problem
associated with the parser.

This one is going on my Python gotchas list. It is really silly from
an end user perspective to have \ not special in raw strings _except_
if it is the last character.

The alternative would have been to offer no way at all for a rawstring
to include its quoting-character. Since rawstrings were designed to
support regular expressions (which never need to end with a backslash),
that was considered a higher cost -- there is no real advantage to
supporting backslash-using Dos/Windows path literals, even though many
Windowsers use rawstrings for those.
Alex

[Blackbird]

pl****@alumni.c altech.edu wrote:

Hi Duncan,

thanks for the reply. I figured that this was a technical problem
associated with the parser.

This one is going on my Python gotchas list. It is really silly from
an end user perspective to have \ not special in raw strings _except_
if it is the last character.

But at the parsing stage, it *is* somewhat special, even if it is not the
last charater. E.g,

a = r'check \' this'
print a

The second quote is not regarded as ending the string literal.

[Steven D'Aprano]

On Sun, 05 Mar 2006 08:27:31 -0800, plahey wrote:

Hi Duncan,

thanks for the reply. I figured that this was a technical problem
associated with the parser.

This one is going on my Python gotchas list. It is really silly from
an end user perspective to have \ not special in raw strings _except_
if it is the last character.

I don't deny that this is a gotcha, but you misunderstand the purpose of
raw strings. They weren't designed so that Windows users could enter
pathnames with backslashes. Raw strings are designed to enter regular
expressions, and for regular expressions, not being able to end a string
with a backslash is not a bug but a feature.

See http://www.ferg.org/projects/python_...ontents_item_2

(and try not to choke on the oh-so-saccharine-sweet cutesy introduction.
Red Ridinghood indeed *wink*)
--
Steven.

[plahey]

Hi Alex,

thanks for the reply. I can see that there is a choice that needed to
be made. Before I was aware of the \ issue I just used (yes it has
come up, but the example is at work) triple quotes to work around the
embedded quote issue:

x=r'''It's like this "c:\blah\" ok?'''
print x
It's like this "c:\blah\" ok?

Am I missing something?

I guess my point is that it is quite surprising that r'\' is not a
legal way to represent a backslash. I look for consistency in the
tools that I work with and this except-for-the-last-character exception
is annoying (I admit it might be a silly use-case but consistency is
consistency).

Again, thanks for taking the time to reply.

Now get back to work on your new Nutshell book :-)

[plahey]

Hi,

thanks for the reply. I was not aware of this in raw strings (and
frankly, don't like it... but who cares about that :-) )

When I needed embedded quotes in a raw string I went the triple quote
route:

a = r'''check \' this'''

which makes more sense to me.

[plahey]

Hi Steven,

thanks for the reply. I was/am aware that raw strings are mainly used
for regular expressions (and franky that was I usually use them for).
I was not aware that they still have "special powers" in raw strings
(thanks for the link!).

This one bit me when I was doing some quick and dirty hacking and a 5
minute task ended up being a much longer adventure becuase of this
(Note to self: don't post when annoyed).

[Alex Martelli]

<pl****@alumni. caltech.edu> wrote:

thanks for the reply. I can see that there is a choice that needed to
be made. Before I was aware of the \ issue I just used (yes it has
come up, but the example is at work) triple quotes to work around the
embedded quote issue:

x=r'''It's like this "c:\blah\" ok?'''
print x
It's like this "c:\blah\" ok?

Am I missing something?
No, triplequotes are perfectly valid.
I guess my point is that it is quite surprising that r'\' is not a
legal way to represent a backslash. I look for consistency in the
tools that I work with and this except-for-the-last-character exception
is annoying (I admit it might be a silly use-case but consistency is
consistency).
Alas, somebody will now quote Emerson at you, I fear;-). Me, I
appreciate consistency and regularity more than most (without making a
fetish of it), but the rule "a rawstring cannot end with an odd number
of backslashes" is perfectly regular and consistent, so I'm not
perturbed by it (it's NOT about the last character, as r'\\' is just
fine even though its last character is a \ -- it's about the parity of
the length of the terminating sequence of backslashes: even is OK [0 is
of course even;-)] and odd is not).
Again, thanks for taking the time to reply.

Now get back to work on your new Nutshell book :-)

Yep, good point!-)
Alex