[Python-Dev] PEP 215 redux: toward a simplified consensus? (original) (raw)

Jeff Epler jepler@unpythonic.dhs.org
Mon, 25 Feb 2002 15:01:06 -0600

On Mon, Feb 25, 2002 at 12:31:55PM -0800, Paul Prescod wrote:

Yuck!

String interopolation should be a compile time action, not an operator. One of the goals, in my mind, is to allow people to string interpolate without knowing what the locals() function does. After all, that function is otherwise useless for most Python programmers (and should probably be moved to an introspection module). Your strategy requires the naive user to learn a) the $ syntax, b) the magic operator syntax and c) the meaning of the locals() function. Plus you've thrown away the idea that interpolation works as it does in the shell or in Perl/Awk/Ruby etc. At that point, in my mind, we're back where we started and should just use %. Well have reinvented it with a few small tweaks. Plus, operator-based evaluation has some security implications that compile time evaluation does not. In particular, if the left-hand thing can be any string then you have the potential of accidentally allowing the user to supply a string that allows him/her to introspect your local variables. That can't happen if the interpolation is done at compile time.

But how do you internationalize your program once you use $-subs? The great strength of %-formats, and the *printf functions that inspired them, are that the interpretation of the format takes place at runtime. (printf has added positional specifiers, spelled like "%1$s", to permit reordering of items in the format, while Python has added key-specifiers, spelled like "%(id)s", but they're about equally powerful)

With %-subs, we can write def gettext(s): """ Return the localized version of s from the message catalog """ return s

def print_chance(who, chance):
print gettext("%(who)s has a %(percent).2f%% chance of surviving") % {
    'who': who,
    'percent': chance * 100}

print_chance("Jeff", 1./3)

I'm not interested in any proposal that turns code that's easy to internationalize (just add calls to gettext(), commonly spelled _(), around each string that needs translating, then fix up the places where the programmer was too clever) into code that's impossible to internationalize by design.

Jeff