Implement a safer and more predictable interpolation approach.

Closes SF bug #511737.

Doc/lib/libcfgparser.tex

@@ -59,6 +59,16 @@ appropriate for the \samp{\%()s} string interpolation.  Note that
 and will override any value provided in \var{defaults}.
 \end{classdesc}
 
+\begin{classdesc}{SafeConfigParser}{\optional{defaults}}
+Derived class of \class{ConfigParser} that implements a more-sane
+variant of the magical interpolation feature.  This implementation is
+more predictable as well.
+% XXX Need to explain what's safer/more predictable about it.
+New applications should prefer this version if they don't need to be
+compatible with older versions of Python.
+\versionadded{2.3}
+\end{classdesc}
+
 \begin{excdesc}{NoSectionError}
 Exception raised when a specified section is not found.
 \end{excdesc}

Lib/ConfigParser.py

@@ -538,3 +538,51 @@ class ConfigParser(RawConfigParser):
         if value.find("%(") != -1:
             raise InterpolationDepthError(option, section, rawval)
         return value
+
+
+class SafeConfigParser(ConfigParser):
+
+    def _interpolate(self, section, option, rawval, vars):
+        # do the string interpolation
+        L = []
+        self._interpolate_some(option, L, rawval, section, vars, 1)
+        return ''.join(L)
+
+    _interpvar_match = re.compile(r"%\(([^)]+)\)s").match
+
+    def _interpolate_some(self, option, accum, rest, section, map, depth):
+        if depth > MAX_INTERPOLATION_DEPTH:
+            raise InterpolationDepthError(option, section, rest)
+        while rest:
+            p = rest.find("%")
+            if p < 0:
+                accum.append(rest)
+                return
+            if p > 0:
+                accum.append(rest[:p])
+                rest = rest[p:]
+            # p is no longer used
+            c = rest[1:2]
+            if c == "%":
+                accum.append("%")
+                rest = rest[2:]
+            elif c == "(":
+                m = self._interpvar_match(rest)
+                if m is None:
+                    raise InterpolationSyntaxError(
+                        "bad interpolation variable syntax at: %r" % rest)
+                var = m.group(1)
+                rest = rest[m.end():]
+                try:
+                    v = map[var]
+                except KeyError:
+                    raise InterpolationError(
+                        "no value found for %r" % var)
+                if "%" in v:
+                    self._interpolate_some(option, accum, v,
+                                           section, map, depth + 1)
+                else:
+                    accum.append(v)
+            else:
+                raise InterpolationSyntaxError(
+                    "'%' must be followed by '%' or '('")

Lib/test/test_cfgparser.py

@@ -289,10 +289,25 @@ class RawConfigParserTestCase(TestCaseBase):
                                  ('name', 'value')])
 
 
+class SafeConfigParserTestCase(ConfigParserTestCase):
+    config_class = ConfigParser.SafeConfigParser
+
+    def test_safe_interpolation(self):
+        # See http://www.python.org/sf/511737
+        cf = self.fromstring("[section]\n"
+                             "option1=xxx\n"
+                             "option2=%(option1)s/xxx\n"
+                             "ok=%(option1)s/%%s\n"
+                             "not_ok=%(option2)s/%%s")
+        self.assertEqual(cf.get("section", "ok"), "xxx/%s")
+        self.assertEqual(cf.get("section", "not_ok"), "xxx/xxx/%s")
+
+
 def test_main():
     suite = unittest.TestSuite()
     suite.addTests([unittest.makeSuite(ConfigParserTestCase),
-                    unittest.makeSuite(RawConfigParserTestCase)])
+                    unittest.makeSuite(RawConfigParserTestCase),
+                    unittest.makeSuite(SafeConfigParserTestCase)])
     test_support.run_suite(suite)
 
 if __name__ == "__main__":