== A Python quoting irritation

I was writing code today where I needed to turn a single backslash
('\') into two backlashes ('\\') in order to quote it (so that the
shell wouldn't eat it, through a combination of annoyances). My
first attempt was:

>  s = s.replace(r"\", r"\\")

To my surprise, this gave me an error, and a peculiar one: Python
reported _SyntaxError: invalid token_ (at the closing ')'.) It took a
bunch of head-scratching to figure out that Python was blowing up on the
first string. I tried it with triple-quoted strings and that didn't work
either (but with a different error).

Ultimately, this is because Python uses backslash in strings for two
separate jobs: quoting end of string characters, and introducing special
characters like _\n_. The former is active all the time; it is only the
latter that is turned off by the _r_ string modifier.

Where things get really confusing is the handling of '_\\_' in _r_
strings. The double-backslash stops the second backslash from escaping
anything it normally would have, but is not turned into '_\_' in the
parsed string; it remains intact as '_\\_'.

So the ultimate answer I wound up with is:

> s = s.replace("\\", r"\\")

You can write the second string as _"\\\\"_, but that sort of thing
annoys the heck out of me so I would up leaving it as an _r_ string.

(This is all actually documented in the [[string literal section
http://docs.python.org/ref/strings.html]] of the reference manual.
If it is covered in the tutorial, I read it sufficiently long ago
to have forgotten it.)

All of this (coupled with wrestling with shell quoting, which is how
I wound up dealing with this) has reminded me of how much I hate
complicated quoting schemes and the need for quoting in general.