Index: Lib/string.py
===================================================================
--- Lib/string.py	(revision 3416)
+++ Lib/string.py	(working copy)
@@ -412,7 +412,132 @@
     for i in range(len(fromstr)):
 	L[fromstr[i]] = tostr[i]
     return joinfields(L, "")
+    
+import re as _re
 
+class _multimap:
+    """Helper class for combining multiple mappings.
+
+    Used by .{safe_,}substitute() to combine the mapping and keyword
+    arguments.
+    """
+    def __init__(self, primary, secondary):
+        self._primary = primary
+        self._secondary = secondary
+
+    def __getitem__(self, key):
+        try:
+            return self._primary[key]
+        except KeyError:
+            return self._secondary[key]
+
+
+class _TemplateMetaclass(type):
+    pattern = r"""
+    %(delim)s(?:
+      (?P<escaped>%(delim)s) |   # Escape sequence of two delimiters
+      (?P<named>%(id)s)      |   # delimiter and a Python identifier
+      {(?P<braced>%(id)s)}   |   # delimiter and a braced identifier
+      (?P<invalid>)              # Other ill-formed delimiter exprs
+    )
+    """
+
+    def __init__(cls, name, bases, dct):
+        super(_TemplateMetaclass, cls).__init__(name, bases, dct)
+        if 'pattern' in dct:
+            pattern = cls.pattern
+        else:
+            pattern = _TemplateMetaclass.pattern % {
+                'delim' : _re.escape(cls.delimiter),
+                'id'    : cls.idpattern,
+                }
+        cls.pattern = _re.compile(pattern, _re.IGNORECASE | _re.VERBOSE)
+
+
+class Template:
+    """A string class for supporting $-substitutions."""
+    __metaclass__ = _TemplateMetaclass
+
+    delimiter = '$'
+    idpattern = r'[_a-z][_a-z0-9]*'
+
+    def __init__(self, template):
+        self.template = template
+
+    # Search for $$, $identifier, ${identifier}, and any bare $'s
+
+    def _invalid(self, mo):
+        i = mo.start('invalid')
+        lines = self.template[:i].splitlines(True)
+        if not lines:
+            colno = 1
+            lineno = 1
+        else:
+            colno = i - len(''.join(lines[:-1]))
+            lineno = len(lines)
+        raise ValueError('Invalid placeholder in string: line %d, col %d' %
+                         (lineno, colno))
+
+    def substitute(self, *args, **kws):
+        if len(args) > 1:
+            raise TypeError('Too many positional arguments')
+        if not args:
+            mapping = kws
+        elif kws:
+            mapping = _multimap(kws, args[0])
+        else:
+            mapping = args[0]
+        # Helper function for .sub()
+        def convert(mo):
+            # Check the most common path first.
+            named = mo.group('named') or mo.group('braced')
+            if named is not None:
+                val = mapping[named]
+                # We use this idiom instead of str() because the latter will
+                # fail if val is a Unicode containing non-ASCII characters.
+                return '%s' % val
+            if mo.group('escaped') is not None:
+                return self.delimiter
+            if mo.group('invalid') is not None:
+                self._invalid(mo)
+            raise ValueError('Unrecognized named group in pattern',
+                             self.pattern)
+        return self.pattern.sub(convert, self.template)
+
+    def safe_substitute(self, *args, **kws):
+        if len(args) > 1:
+            raise TypeError('Too many positional arguments')
+        if not args:
+            mapping = kws
+        elif kws:
+            mapping = _multimap(kws, args[0])
+        else:
+            mapping = args[0]
+        # Helper function for .sub()
+        def convert(mo):
+            named = mo.group('named')
+            if named is not None:
+                try:
+                    # We use this idiom instead of str() because the latter
+                    # will fail if val is a Unicode containing non-ASCII
+                    return '%s' % mapping[named]
+                except KeyError:
+                    return self.delimiter + named
+            braced = mo.group('braced')
+            if braced is not None:
+                try:
+                    return '%s' % mapping[braced]
+                except KeyError:
+                    return self.delimiter + '{' + braced + '}'
+            if mo.group('escaped') is not None:
+                return self.delimiter
+            if mo.group('invalid') is not None:
+                return self.delimiter
+            raise ValueError('Unrecognized named group in pattern',
+                             self.pattern)
+        return self.pattern.sub(convert, self.template)
+
+
 # Substring replacement (global)
 def replace(s, old, new, maxsplit=-1):
     """replace (str, old, new[, maxsplit]) -> string