SF patch #1056967, changes the semantics of Template.safe_substitute() to not

raise a ValueError for dangling delimiters (the delimiter itself is returned).

Doc/lib/libstring.tex

@@ -131,8 +131,16 @@ precedence.
 Like \method{substitute()}, except that if placeholders are missing from
 \var{mapping} and \var{kws}, instead of raising a \exception{KeyError}
 exception, the original placeholder will appear in the resulting string
-intact.  Note that other exceptions may still be raised, including
-\exception{ValueError} as described above.
+intact.  Also, unlike with \method{substitute()}, any other appearances of the
+\samp{\$} will simply return \samp{\$} instead of raising
+\exception{ValueError}.
+
+While other exceptions may still occur, this method is called ``safe'' because
+substitutions always tries to return a usable string instead of raising an
+exception.  In another sense, \method{safe_substitute()} may be anything other
+than safe, since it will silently ignore malformed templates containing
+dangling delimiters, unmatched braces, or placeholders that are not valid
+Python identifiers.
 \end{methoddesc}
 
 \class{Template} instances also provide one public data attribute:

Lib/string.py

@@ -199,7 +199,7 @@ class Template:
             if mo.group('escaped') is not None:
                 return self.delimiter
             if mo.group('invalid') is not None:
-                self._invalid(mo)
+                return self.delimiter
             raise ValueError('Unrecognized named group in pattern',
                              self.pattern)
         return self.pattern.sub(convert, self.template)

Lib/test/test_pep292.py

@@ -163,20 +163,19 @@ class TestTemplate(unittest.TestCase):
         raises(TypeError, s.safe_substitute, d, {})
 
     def test_delimiter_override(self):
+        eq = self.assertEqual
+        raises = self.assertRaises
         class AmpersandTemplate(Template):
             delimiter = '&'
         s = AmpersandTemplate('this &gift is for &{who} &&')
-        self.assertEqual(s.substitute(gift='bud', who='you'),
-                         'this bud is for you &')
-        self.assertRaises(KeyError, s.substitute)
-        self.assertEqual(s.safe_substitute(gift='bud', who='you'),
-                         'this bud is for you &')
-        self.assertEqual(s.safe_substitute(),
-                         'this &gift is for &{who} &')
+        eq(s.substitute(gift='bud', who='you'), 'this bud is for you &')
+        raises(KeyError, s.substitute)
+        eq(s.safe_substitute(gift='bud', who='you'), 'this bud is for you &')
+        eq(s.safe_substitute(), 'this &gift is for &{who} &')
         s = AmpersandTemplate('this &gift is for &{who} &')
-        self.assertRaises(ValueError, s.substitute,
-                          dict(gift='bud', who='you'))
-        self.assertRaises(ValueError, s.safe_substitute)
+        raises(ValueError, s.substitute, dict(gift='bud', who='you'))
+        eq(s.safe_substitute(), 'this &gift is for &{who} &')
+
 
 def test_main():
     from test import test_support

Misc/NEWS

@@ -56,6 +56,10 @@ Extension Modules
 Library
 -------
 
+- Patch #1056967 changes the semantics of Template.safe_substitute() so that
+  no ValueError is raised on an 'invalid' match group.  Now the delimiter is
+  returned.
+
 - Bug #1052503 pdb.runcall() was not passing along keyword arguments.
 
 - Bug #902037: XML.sax.saxutils.prepare_input_source() now combines relative