[Python-ideas] Custom string prefixes

[Göktuğ Kayaalp]

Hi,

I wanted to share this idea of (not possibly only) mine with Python
Core developers.

I think it would add some significant flexibility to Python to let users
define custom string prefixes. What I mean by a string prefix is,
a letter prefixing the string literal, modifying the behavior of it,
e.g.

     >>> r"^\?\? (?P<branch>.*)$"

is a so-called /raw string/ which does not interpret character escapes.
I would suggest that providing ability to define custom string prefixes
would benefit the programmer, especially enrich Python based DSLs.

Let me provide some examples:

  - I know that there is a library called `decimal`, which provides
    facilities for finer floating point arithmetic. A `Decimal`
    class is used to express these numbers and operations, resulting in

        >>> decimal.Decimal ("1.6e-9") * decimal.Decimal ("1.0e9")

    which is a little bit messy. This can easily be cured by

        >>> from decimal import Decimal as D
        >>> D ("1.6e-9") * D ("1.0e9")

    but I'd enounce that the following is more concise and readable:

        >>> D"1.6e-9" * D"1.0e9"

    with removed parens.

  - I use a little program I wrote instead of make and rake for my daily
    computer usage called poke[*], where I do a lot of string
    interpolation.

        from poke.utils import shell, build as _build
        OUTDIR, PROGRAM = "out", "lsvirtues"

        def build ():
          t0 = _build.CompilationTask (
                 ["laziness.c", "hubris.c", "impatience.c"],
                 "{outdir}/{progn}".format(outdir=OUTDIR, progn=PROGRAM),
                 "gcc -Wall -o {outfile} {infiles}")
          t0.commit ()

        def clean ():
          shell.sh ("rm -fr {0} *.o .pokedb".format (OUTDIR))

    Even though `str.format (*, **)` is cool, I think using an
    'interpolated string' prefix can help clean up stuff a little bit:
    
       # ...
       def build ():
         t0 = _build.CompilationTask ([...], I"{OUTDIR}/{progn}", ...)
       
       def clean ():
         shell.sh (I"rm -fr {OUTDIR} *.o .pokedb")
    
I thought of subclassing the `str` and overloading some unary operators,
e.g. "~", but, I believe this is not very well suited because

    - still requires some mess: ~s("hello {name}")
    - or, a monkey-patched `str`: ~"we should{should_we} do that"
    - and, a tilde is not as meaningful as a letter.

One can find many other uses too, which saves some redundant method
calls and stuff for a particular task. And such an addition to the
language would be compatible too, in the sense that it will be possible
to run older scripts on a runtime with this implemented.

I'm looking forward to your criticisms and advices. I've searched this
online and asked in the chatroom (#python) and I'm nearly sure that I'm
not asking for a feature that is already present. Being a beginner, I
can say that I'm kind of nervous to post here, where really experienced
people discuss the features of an internationally-adopted language.

[*] https://github.com/gkya/poke

Greetings,

       Göktuğ.

-- 
Göktuğ Kayaalp <goktug.kayaalp at gmail.com>