Enumerating formatting strings

I was messing about with formatting and realized that the right kind of object could quite easily tell me exactly what accesses are made to the mapping in a string % mapping operation. This is a fairly well-known technique, modified to tell me what keys would need to be present in any mapping used with the format.

class Everything:
    def __init__(self, format="%s", discover=False):
        self.names = {}
        self.values = []
        self.format=format
        self.discover = discover
    def __getitem__(self, key):
        x = self.format % key
        if self.discover:
            self.names[key] = self.names.get(key, 0) + 1
        return x
    def nameList(self):
         if self.names:
             return ["%-20s %d" % i for i in self.names.items()]
         else:
             return self.values
    def __getattr__(self, name):
        print "Attribute", name, "requested"
        return None
    def __repr__(self):
        return "<Everything object at 0x%x>"  % id(self)

def nameCount(template):
    et = Everything(discover=True)
    p = template % et
    nlst = et.nameList()
    nlst.sort()
    return nlst

for s in nameCount("%(name)s %(value)s %(name)s"):
    print s

The result of this effort is:

name                 2
value                1

I've been wondering whether it's possible to perform a similar analysis on non-mapping-type format strings, so as to know how long a tuple to provide, or whether I'd be forced to lexical analysis of the form string.

regards
 Steve
--
Steve Holden        +1 703 861 4237  +1 800 494 3119
Holden Web LLC             http://www.holdenweb.com/
Python Web Programming  http://pydish.holdenweb.com/

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