var_expand.py

from pyparsing import *
from string import printable

data = "${foo} ${${bar}} whee ${ bar ${baz ${meh} moo}}"
datastore = {
    "foo": "fooval",
    "bar": "barval",
    "barval": "barvalval",
    "meh": "MEH",
    "baz MEH moo": "heh.",
    " bar heh.": "dance!",
}
expanded_data = "fooval barvalval whee dance!"

def getVar(tokens):
    var = tokens[0]
    val = "".join(var.asList())
    if val in datastore:
        tokens[0] = datastore[val]

content = Combine(OneOrMore(~Literal("${") + ~Literal("}") +
                            Word(printable, exact=1)))
content.leaveWhitespace()
var = nestedExpr("${", "}", ignoreExpr=None, content=content)
var.setParseAction(getVar)

newdata = var.transformString(data)
assert(newdata == expanded_data)