[Python-Dev] features i'd like [Python 3000?] ... #4: interpolated strings ala perl

Josiah Carlson jcarlson at uci.edu
Mon Dec 4 09:32:46 CET 2006

Ben Wing <ben at 666.com> wrote:
> sorry to be casting multiple ideas at once to the list.  i've been 
> looking into other languages recently and reading the recent PEP's and 
> such and it's helped crystallize ideas about what could be better about 
> python.
> i see in PEP 3101 that there's some work going on to fix up the string 
> formatting capabilities of python.  it looks good to me but it still 
> doesn't really address the lack of a simple interpolated string 
> mechanism, as in perl or ruby.  i find myself constantly writing stuff like
> text="Family: %s" % self.name
> maybe_errout(i"[title], line [lineno]: [errstr]\n")

This can be done now via:
    maybe_errout("%(title)s, line %(lineno)i: %(errstr)s\n"%locals())

>     def __str__(self):
>         return i"CCGFeatval([self.name], parents=[self.parents], 
> licensing=[self.licensing])"

With the proper mapping, this is trivial...

class namelookup:
    def __init__(self, namespace):
        self.ns = namespace
    def __getitem__(self, name):
        ns = self.ns
        names = name.split('.')
            if names[0] in ns:
                ns = ns[names[0]]
                names = names[1:]
        except IndexError:
            raise KeyError("bad namespace")
        for n in names:
            if hasattr(ns, n):
                ns = getattr(ns, n)
                return "<name not found>"
        return ns

>>> class foo:
...     a = 1
...     b = 2
>>> foo = foo()
>>> print "%(foo.b)i + %(foo.a)i"%namelookup(locals())
2 + 1

While I understand the desire for better string interpolation, many of
your needs can be covered with pre-existing string formatting.  You can
even write a handler for "{2} and {1} or {3}" as "%(2)s and %(1)s or %
(3)s"%positional(a, b, c). Toss it into site.py, and you can use it

If you want this to be fully in regards to Py3k, post on the py3k
mailing list: python-3000 at python.org

 - Josiah

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