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Thu Oct 23 10:46:48 EDT 2003 · https://mail.python.org/pipermail/python-dev/2003-October/039369.html

On Thu, 2003-10-23 at 00:16, Guido van Rossum wrote:

> There have been many proposals in this area, even a PEP (PEP 215,
> which I don't like that much, despite its use of $).

And PEP 292, which I probably should update.

I should mention that $string substitutions are optional in Mailman 2.1,
but they will be the only way to do it in Mailman 3.  I've played a lot
with various implementations of this idea, and below is the one I've
currently settled on.  Not all of the semantics may be perfect for core
Python (i.e. never throw a KeyError), but this is all doable in modern
Python, and for user-exposed templates, gets a +1000 in my book.

>>> s = dstring('${person} lives in $where and owes me $$${amount}')
>>> d = safedict(person='Guido', where='California', amount='1,000,000')
>>> print s % d
Guido lives in California and owes me $1,000,000
>>> d = safedict(person='Tim', amount=.13)
>>> print s % d
Tim lives in ${where} and owes me $0.13

-Barry

import re
# Search for $$, $identifier, or ${identifier}
dre = re.compile(r'(\${2})|\$([_a-z]\w*)|\${([_a-z]\w*)}', re.IGNORECASE)

EMPTYSTRING = ''

class dstring(unicode):
    def __new__(cls, ustr):
        ustr = ustr.replace('%', '%%')
        parts = dre.split(ustr)
        for i in range(1, len(parts), 4):
            if parts[i] is not None:
                parts[i] = '$'
            elif parts[i+1] is not None:
                parts[i+1] = '%(' + parts[i+1] + ')s'
            else:
                parts[i+2] = '%(' + parts[i+2] + ')s'
        return unicode.__new__(cls, EMPTYSTRING.join(filter(None, parts)))

class safedict(dict):
    """Dictionary which returns a default value for unknown keys."""
    def __getitem__(self, key):
        try:
            return super(safedict, self).__getitem__(key)
        except KeyError:
            return '${%s}' % key

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[Python-Dev] Can we please have a better dict interpolation syntax?