Conditional formatting in Python

Greately inspired by $if(...) conditions in Winamp Advanced Title Formatting, I wanted to add something like this to Python.

Script uses quite a lot of regexps, but looks right. Nested conditions are not supported, though. Also note that about half of the code is comments :-)

from collections import defaultdict
from string import Formatter
import re

# converts a dict to defaultdict:
# where missing values are also nesteddefaultdict:
#   a = nesteddefaultdict()
#   a['b']['c'] = 'd'
#   a == {'b':{'c':'d'}}
# but that's true only on top level (it's a 'shallow copy'):
#   a = nesteddefaultdict({'b': {'c':'d'}})
#   print a['b']['d'] <= fails :(
#
# replaceable with an empty string:
#   str(a) = ''
# useful when:
#   a = nesteddefaultdict({'foo': 1})
#   print a['bar'] => prints '' instead of failing
def nesteddefaultdict(basedict={}):
    class d2(defaultdict):
        def __str__(self):
            return ''
    result = d2(nesteddefaultdict)
    result.update(basedict)
    return result

# format a string, almost like 'str'.format() does.
# Args:
#   text - format string
#   data - dict, defaultdict or nesteddefaultdict
#
# in following examples we assume that:
#   data=nesteddefaultdict({'foo':1})
# uses {keyword} expansions, like this:
#   format('foo equals {foo}', data) => 'foo equals 1'
# 'safe' substitution is a feature of defaultdict:
#   format('bar equals {bar}', data) => 'bar equals '
# same for [indexes]:
#   format('a is {a[foo]} and {a[bar]}', {'a':data}) => 'a is 1 and '
# ===Conditional formatting===
# {{?...?}} -- shows content only if it has non-empty {} expansions:
#   format('{{? foo={foo} ?}}', data) => ' foo=1 '
#   format('{{? bar={bar} ?}}', data) => ''
# {{?...?...|...?}} -- if-then-else ternary operator:
#   shows second group if after substitution first group has non-space character, third group otherwise:
#   format('a has: {{? {a[foo]} ? foo={a[foo]} | no foo ?}}', {'a':data}) => 'a has:  foo=1 '
#   format('a has: {{? {a[bar]} ? bar={a[bar]} | no bar ?}}', {'a':data}) => 'a has:  no bar '
# {{?...==...?...|...?}} -- if-then-else ternary operator with comparison
# {{?...!=...?...|...?}} -- if-then-else ternary operator with comparison
#   format('{{? {foo}==1 ? foo equals one | different ?}}', data) => ' foo equals one '
#   format('{{? {bar}!= ? bar is non-empty | bar is empty ?}}', data) => ' bar is empty '
def format(text, data):
    dict_with_nothing=nesteddefaultdict()
    # {{?a?}}
    text = re.split('{{\?([^?|]*)\?}}', text)
    # delete those groups where data has no visible effect
    for i in range(1, len(text), 2):
        group_with_data = Formatter().vformat(text[i],[],data)
        group_with_nothing = Formatter().vformat(text[i],[],dict_with_nothing)
        if group_with_data == group_with_nothing:
            text[i] = ''
    text = ''.join(text)
    # apply all format options
    text = Formatter().vformat(text,[],data)
    # {{?a[!=]b?c|d?}}
    text = re.split('{\?([^?=]*)([=!])=([^?]*)\?([^|?]*)\|([^?]*)\?}', text)
    # select one of two groups (i+3 or i+4) based on (i+1) comparison of others (i and i+2)
    for i in range(1, len(text), 6):
        if (text[i].strip()==text[i+2].strip()) == (text[i+1]=='='):
            text[i+4] = ''
        else:
            text[i+3] = ''
        text[i] = text[i+1] = text[i+2] = ''
    text = ''.join(text)
    # {{?a?b|c?}}
    text = re.split('{\?([^?]*)\?([^|?]*)\|([^?]*)\?}', text)
    # select one of two groups (i+1 or i+2) based on third one (i)
    for i in range(1, len(text), 4):
        if re.search('\S', text[i]):
            text[i+2] = ''
        else:
            text[i+1] = ''
        text[i] = ''
    text = ''.join(text)
    return text

if __name__ == "__main__":
    a = nesteddefaultdict()
    a['b']['c'] = 'd'
    print a == {'b':{'c':'d'}}

    data = nesteddefaultdict({'foo':1})
    print format('"foo equals {foo}"', data), '"foo equals 1"'
    print format('"bar equals {bar}"', data), '"bar equals "'
    print format('"a is {a[foo]} and {a[bar]}"', {'a':data}), '"a is 1 and "'
    print format('"{{? foo={foo} ?}}"', data), '" foo=1 "'
    print format('"{{? bar={bar} ?}}"', data), '""'
    print format('"a has: {{? {a[foo]} ? foo={a[foo]} | no foo ?}}"', {'a':data}), '"a has:  foo=1 "'
    print format('"a has: {{? {a[bar]} ? bar={a[bar]} | no bar ?}}"', {'a':data}), '"a has:  no bar "'
    print format('"{{? {foo}==1 ? foo equals one | different ?}}"', data), '" foo equals one "'
    print format('"{{? {bar}!= ? bar is non-empty | bar is empty ?}}"', data), '" bar is empty "'