Simple template replacement function

substitute.py

import re

def make_replacer(d): 
    "bind dictionary to replacement function"
    return lambda m: d.get(m.group(1), m.group(0))

def subs(template, obj):
    "substitute values from obj into ${vars} in template"
    repl = make_replacer(obj)
    return re.sub("""\$\{(\w+)\}""", repl, template)

# to use:
if __name__ == "__main__":
    subs("Say ${greeting} to ${place}", {"greeting": "HELLO", "place": "WORLD"})