[Python-Dev] PEP 292, Simpler String Substitutions

[Barry A. Warsaw]

>>>>> "RH" == Raymond Hettinger <python@rcn.com> writes:

    >> 1. $$ is an escape;
    >> it is replaced with a single $

    RH> Hmm, some strings (at least in the spam I receive) contain
    RH> $$$$$$.  How about ${$}?

How often will you be interpolating into spam?  Sounds like it could
get messy. ;)

    >> Handling Missing Keys
    >> What should happen when one of the substitution keys is missing
    >> from the mapping (or the locals/globals namespace if no
    >> argument is given)?  There are two possibilities: - We can
    >> simply allow the exception (likely a NameError or KeyError) to
    >> propagate.  - We can return the original substitution
    >> placeholder unchanged.

    RH> And/Or,
    RH>        - Leave placeholder unchanged unless default argument
    RH> supplied: mystr.sub(mydict, undefined='***') # Fill unknowns
    RH> with
    RH> stars
    RH> And/Or,
    RH>        - Raise an exception only if specified:
    RH>                   mystr.sub(mydict, undefined=NameError)
    RH> And/Or
    RH>         - Return a count of the number of missed substitutions:
    RH>                   nummisses = mystr.sub(mydict)

I /really/ dislike interfaces that raise an exception or don't, based
on an argument to the function.  Returning the number of missed
substitutions doesn't seem useful, but could be done with a regexp.
Filling unknowns with stars could just as easily be done with a
different safedict wrapper.

The specific issue is when using locals+globals.  In that case, it's
seems like the problem is clearly a programming bug, so the exception
should be raised.

    >> BDFL proto-pronouncement: It should always raise a NameError
    >> when the key is missing.  There may not be sufficient use case
    >> for soft failures in the no-argument version.

    RH> I had written a minature mail-merge program and learned that
    RH> the NameError approach is a PITA.  It makes sense if the
    RH> mapping is defined inside the program; however, externally
    RH> supplied mappings (like a mergelist) can be expected to have
    RH> "holes" and launching exceptions makes it harder to recover
    RH> than having a default behavior.  The best existing Python
    RH> comparison is the str.replace() method which does not bomb-out
    RH> when the target string is not found.

I agree that certain use cases make the exception problematic.  Think
a program that uses a template entered remotely through the web.  That
template could have misspellings in the variable substitutions.  In
that case I think you'd like to carry on as best you can, by returning
a string with the bogus placeholders still in the string.

-Barry