1411 

  1412 def interpolate(prefix, mapping, s, fn=None):

  1413     """Return the result of interpolating items in the mapping into string s.

  1414 

  1415     prefix is a single character string, or a two character string with

  1416     a backslash as the first character if the prefix needs to be escaped in

  1417     a regular expression.

  1418 

  1419     fn is an optional function that will be applied to the replacement text

  1420     just before replacement.

  1421     """

  1422     fn = fn or (lambda s: s)

  1423     r = re.compile(r'%s(%s)' % (prefix, '|'.join(mapping.keys())))

  1424     return r.sub(lambda x: fn(mapping[x.group()[1:]]), s)

  1425