Simple question - end a raw string with a single backslash ?

[Peter J. Holzer]

On 2020-10-17 21:03:26 -0000, Mladen Gogala via Python-list wrote:
> On Thu, 15 Oct 2020 21:30:15 +0000, Stefan Ram wrote:
> > Tony Flury <tony.flury at btinternet.com> writes:
> >> >>> a = r'end' + chr(92)
> > 
> >   Or maybe,
> > 
> > a = r'''
> > end\
> > '''[ 1: -1 ]
> > 
> >   ? The first and the last line are messy, but in the middle,
> >   the intended string is clearly visible.
> 
> 
> You can use perl module for python.

Ah, I see, that the sillyness of Perl's grammar-altering modules (which
let you write Perl in Latin (with proper declensions and conjugations,
of course) or Chinese) has found its way to Python :-)


> It is installable by pip.
> Perl has no problems with handling backslashes.
> 
> https://pypi.org/project/perl/
> 
> What you need is something like this:
> 
> mgogala at umajor:~$ perl -e '$a="abcd";$a=~s/$/\\/; print "$a\n";'
> abcd\
> 
> Python has a problem with backslashes:
> 
> Python 3.8.5 (default, Jul 28 2020, 12:59:40) 
> [GCC 9.3.0] on linux
> Type "help", "copyright", "credits" or "license" for more information.
> >>> import re
> >>> a="abcd"
> >>> b=re.sub('$',chr(28),a)
> >>> b
> 'abcd\x1c'
> 
> >>> b=re.sub('$',chr(0x41),a)
> >>> b
> 'abcdA'
> >>> b=re.sub('$','Z',a)
> >>> b
> 'abcdZ'
> >>> b=re.sub('$',chr(0x1C),a)
> >>> b
> 'abcd\x1c'
> >>> 
> 
> Any other character is shown as it should be, except backslash.

0x1C isn't a backslash, it's a control character (FS - File Separator).

0x5C is a backslash, but of course that doesn't work here either,
because the second argument to re.sub isn't a simple string: It contains
escape sequences to be interpreted, and a single \ isn't well-formed
(note that you doubled the \ in your Perl example, too).

b=re.sub('$', '\\\\', a)
or
b=re.sub('$', chr(0x5C)+chr(0x5C), a)

works just fine, as does 

b = a + chr(0x5C)

In all these cases, 
print(b) 
prints
abcd\
(with a single backslash at the end)

But simply typing "b" at the REPL produces
'abcd\\'
because prints the __repr__() of an object. Note that it also prints
single quotes, which are also not part of the string.

        hp

-- 
   _  | Peter J. Holzer    | Story must make more sense than reality.
|_|_) |                    |
| |   | hjp at hjp.at         |    -- Charles Stross, "Creative writing
__/   | http://www.hjp.at/ |       challenge!"
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