Slightly revised version of patch #1538956:

Replace UnicodeDecodeErrors raised during == and !=
compares of Unicode and other objects with a new
UnicodeWarning.

All other comparisons continue to raise exceptions.
Exceptions other than UnicodeDecodeErrors are also left
untouched.

Doc/api/concrete.tex

@@ -1560,6 +1560,31 @@ They all return \NULL{} or \code{-1} if an exception occurs.
   greater than, respectively.
 \end{cfuncdesc}
 
+\begin{cfuncdesc}{int}{PyUnicode_RichCompare}{PyObject *left, 
+                                              PyObject *right, 
+                                              int op}
+
+% This entry could use some polishing - my TeX is too
+% rusty these days... (MAL)
+
+  Rich compare two strings and return one of the following:
+\begin{verbatim}
+   - NULL in case an exception was raised
+   - Py_True or Py_False for successfuly comparisons
+   - Py_NotImplemented in case the type combination is unknown
+\end{verbatim}
+
+   Note that Py_EQ and Py_NE comparisons can cause a UnicodeWarning in
+   case the conversion of the arguments to Unicode fails with a
+   UnicodeDecodeError.
+
+   Possible values for \var{op}:
+\begin{verbatim}
+     Py_GT, Py_GE, Py_EQ, Py_NE, Py_LT, Py_LE
+\end{verbatim}
+
+\end{cfuncdesc}
+
 \begin{cfuncdesc}{PyObject*}{PyUnicode_Format}{PyObject *format,
                                               PyObject *args}
   Return a new string object from \var{format} and \var{args}; this

Doc/api/exceptions.tex

@@ -288,10 +288,11 @@ for each thread.
   names are \samp{PyExc_} followed by the Python exception name.
   These have the type \ctype{PyObject*}; they are all class objects.
   Their names are \cdata{PyExc_Warning}, \cdata{PyExc_UserWarning},
-  \cdata{PyExc_DeprecationWarning}, \cdata{PyExc_SyntaxWarning},
+  \cdata{PyExc_UnicodeWarning}, \cdata{PyExc_DeprecationWarning},
-  \cdata{PyExc_RuntimeWarning}, and \cdata{PyExc_FutureWarning}.
+  \cdata{PyExc_SyntaxWarning}, \cdata{PyExc_RuntimeWarning}, and
-  \cdata{PyExc_Warning} is a subclass of \cdata{PyExc_Exception}; the
+  \cdata{PyExc_FutureWarning}.  \cdata{PyExc_Warning} is a subclass of
-  other warning categories are subclasses of \cdata{PyExc_Warning}.
+  \cdata{PyExc_Exception}; the other warning categories are subclasses
+  of \cdata{PyExc_Warning}.
 
   For information about warning control, see the documentation for the
   \module{warnings} module and the \programopt{-W} option in the

Doc/lib/libexcs.tex

@@ -456,6 +456,11 @@ Base class for warnings about probable mistakes in module imports.
 \versionadded{2.5}
 \end{excdesc}
 
+\begin{excdesc}{UnicodeWarning}
+Base class for warnings related to Unicode.
+\versionadded{2.5}
+\end{excdesc}
+
 The class hierarchy for built-in exceptions is:
 
 \verbatiminput{../../Lib/test/exception_hierarchy.txt}

Doc/lib/libwarnings.tex

@@ -76,6 +76,9 @@ features that will be deprecated in the future (ignored by default).}
 
 \lineii{ImportWarning}{Base category for warnings triggered during the
 process of importing a module (ignored by default).}
+
+\lineii{UnicodeWarning}{Base category for warnings related to Unicode.}
+
 \end{tableii}
 
 While these are technically built-in exceptions, they are documented

Include/pyerrors.h

@@ -173,6 +173,7 @@ PyAPI_DATA(PyObject *) PyExc_SyntaxWarning;
 PyAPI_DATA(PyObject *) PyExc_RuntimeWarning;
 PyAPI_DATA(PyObject *) PyExc_FutureWarning;
 PyAPI_DATA(PyObject *) PyExc_ImportWarning;
+PyAPI_DATA(PyObject *) PyExc_UnicodeWarning;
 
 
 /* Convenience functions */

Include/unicodeobject.h

@@ -189,6 +189,7 @@ typedef PY_UNICODE_TYPE Py_UNICODE;
 # define PyUnicode_RSplit PyUnicodeUCS2_RSplit
 # define PyUnicode_Replace PyUnicodeUCS2_Replace
 # define PyUnicode_Resize PyUnicodeUCS2_Resize
+# define PyUnicode_RichCompare PyUnicodeUCS2_RichCompare
 # define PyUnicode_SetDefaultEncoding PyUnicodeUCS2_SetDefaultEncoding
 # define PyUnicode_Split PyUnicodeUCS2_Split
 # define PyUnicode_Splitlines PyUnicodeUCS2_Splitlines
@@ -266,6 +267,7 @@ typedef PY_UNICODE_TYPE Py_UNICODE;
 # define PyUnicode_RSplit PyUnicodeUCS4_RSplit
 # define PyUnicode_Replace PyUnicodeUCS4_Replace
 # define PyUnicode_Resize PyUnicodeUCS4_Resize
+# define PyUnicode_RichCompare PyUnicodeUCS4_RichCompare
 # define PyUnicode_SetDefaultEncoding PyUnicodeUCS4_SetDefaultEncoding
 # define PyUnicode_Split PyUnicodeUCS4_Split
 # define PyUnicode_Splitlines PyUnicodeUCS4_Splitlines
@@ -1139,6 +1141,28 @@ PyAPI_FUNC(int) PyUnicode_Compare(
     PyObject *right		/* Right string */
     );
 
+/* Rich compare two strings and return one of the following:
+
+   - NULL in case an exception was raised
+   - Py_True or Py_False for successfuly comparisons
+   - Py_NotImplemented in case the type combination is unknown
+
+   Note that Py_EQ and Py_NE comparisons can cause a UnicodeWarning in
+   case the conversion of the arguments to Unicode fails with a
+   UnicodeDecodeError.
+
+   Possible values for op:
+
+     Py_GT, Py_GE, Py_EQ, Py_NE, Py_LT, Py_LE
+
+*/
+
+PyAPI_FUNC(PyObject *) PyUnicode_RichCompare(
+    PyObject *left,		/* Left string */ 
+    PyObject *right,		/* Right string */
+    int op			/* Operation: Py_EQ, Py_NE, Py_GT, etc. */
+    );
+
 /* Apply a argument tuple or dictionary to a format string and return
    the resulting Unicode string. */
 

Lib/test/exception_hierarchy.txt

@@ -45,3 +45,4 @@ BaseException
            +-- UserWarning
            +-- FutureWarning
 	   +-- ImportWarning
+	   +-- UnicodeWarning

Misc/NEWS

@@ -12,18 +12,18 @@ What's New in Python 2.5 release candidate 1?
 Core and builtins
 -----------------
 
-- Fix segfault when doing string formatting on subclasses of long.
+- Unicode objects will no longer raise an exception when being
+  compared equal or unequal to a string and causing a
+  UnicodeDecodeError exception, e.g. as result of a decoding failure.
 
-- Fix bug related to __len__ functions using values > 2**32 on 64-bit machines
+  Instead, the equal (==) and unequal (!=) comparison operators will
-  with new-style classes.
+  now issue a UnicodeWarning and interpret the two objects as
-  
+  unequal. The UnicodeWarning can be filtered as desired using
-- Fix bug related to __len__ functions returning negative values with
+  the warning framework, e.g. silenced completely, turned into an
-  classic classes.
+  exception, logged, etc.
-  
+
-- Patch #1538606, Fix __index__() clipping.  There were some problems
+  Note that compare operators other than equal and unequal will still
-  discovered with the API and how integers that didn't fit into Py_ssize_t
+  raise UnicodeDecodeError exceptions as they've always done.
-  were handled.  This patch attempts to provide enough alternatives
-  to effectively use __index__.
 
 - Bug #1536021: __hash__ may now return long int; the final hash
   value is obtained by invoking hash on the long int.
@@ -99,6 +99,8 @@ Build
 C API
 -----
 
+- New API for Unicode rich comparisons: PyUnicode_RichCompare()
+
 - Bug #1069160.  Internal correctness changes were made to
   ``PyThreadState_SetAsyncExc()``.  A test case was added, and
   the documentation was changed to state that the return value

Objects/exceptions.c

@@ -1948,6 +1948,14 @@ SimpleExtendsException(PyExc_Warning, ImportWarning,
           "Base class for warnings about probable mistakes in module imports");
 
 
+/*
+ *    UnicodeWarning extends Warning
+ */
+SimpleExtendsException(PyExc_Warning, UnicodeWarning,
+    "Base class for warnings about Unicode related problems, mostly\n"
+    "related to conversion problems.");
+
+
 /* Pre-computed MemoryError instance.  Best to create this as early as
  * possible and not wait until a MemoryError is actually raised!
  */
@@ -2048,6 +2056,7 @@ _PyExc_Init(void)
     PRE_INIT(RuntimeWarning)
     PRE_INIT(FutureWarning)
     PRE_INIT(ImportWarning)
+    PRE_INIT(UnicodeWarning)
 
     m = Py_InitModule4("exceptions", functions, exceptions_doc,
         (PyObject *)NULL, PYTHON_API_VERSION);
@@ -2113,6 +2122,7 @@ _PyExc_Init(void)
     POST_INIT(RuntimeWarning)
     POST_INIT(FutureWarning)
     POST_INIT(ImportWarning)
+    POST_INIT(UnicodeWarning)
 
     PyExc_MemoryErrorInst = BaseException_new(&_PyExc_MemoryError, NULL, NULL);
     if (!PyExc_MemoryErrorInst)

Objects/object.c

@@ -731,23 +731,6 @@ default_3way_compare(PyObject *v, PyObject *w)
 		return (vv < ww) ? -1 : (vv > ww) ? 1 : 0;
 	}
 
-#ifdef Py_USING_UNICODE
-	/* Special case for Unicode */
-	if (PyUnicode_Check(v) || PyUnicode_Check(w)) {
-		c = PyUnicode_Compare(v, w);
-		if (!PyErr_Occurred())
-			return c;
-		/* TypeErrors are ignored: if Unicode coercion fails due
-		   to one of the arguments not having the right type, we
-		   continue as defined by the coercion protocol (see
-		   above).  Luckily, decoding errors are reported as
-		   ValueErrors and are not masked by this technique. */
-		if (!PyErr_ExceptionMatches(PyExc_TypeError))
-			return -2;
-		PyErr_Clear();
-	}
-#endif
-
 	/* None is smaller than anything */
 	if (v == Py_None)
 		return -1;

Objects/unicodeobject.c

@@ -5405,6 +5405,82 @@ onError:
     return -1;
 }
 
+PyObject *PyUnicode_RichCompare(PyObject *left,
+                                PyObject *right,
+                                int op)
+{
+    int result;
+
+    result = PyUnicode_Compare(left, right);
+    if (result == -1 && PyErr_Occurred())
+        goto onError;
+
+    /* Convert the return value to a Boolean */
+    switch (op) {
+    case Py_EQ:
+        result = (result == 0);
+        break;
+    case Py_NE:
+        result = (result != 0);
+        break;
+    case Py_LE:
+        result = (result <= 0);
+        break;
+    case Py_GE:
+        result = (result >= 0);
+        break;
+    case Py_LT:
+        result = (result == -1);
+        break;
+    case Py_GT:
+        result = (result == 1);
+        break;
+    }
+    return PyBool_FromLong(result);
+
+ onError:
+
+    /* Standard case
+
+       Type errors mean that PyUnicode_FromObject() could not convert
+       one of the arguments (usually the right hand side) to Unicode,
+       ie. we can't handle the comparison request. However, it is
+       possible that the other object knows a comparison method, which
+       is why we return Py_NotImplemented to give the other object a
+       chance.
+
+    */
+    if (PyErr_ExceptionMatches(PyExc_TypeError)) {
+        PyErr_Clear();
+        Py_INCREF(Py_NotImplemented);
+        return Py_NotImplemented;
+    }
+    if (op != Py_EQ && op != Py_NE)
+        return NULL;
+
+    /* Equality comparison.
+
+       This is a special case: we silence any PyExc_UnicodeDecodeError
+       and instead turn it into a PyErr_UnicodeWarning.
+
+    */
+    if (!PyErr_ExceptionMatches(PyExc_UnicodeDecodeError))
+        return NULL;
+    PyErr_Clear();
+    if (PyErr_Warn(PyExc_UnicodeWarning, 
+                   (op == Py_EQ) ? 
+                   "Unicode equal comparison "
+                   "failed to convert both arguments to Unicode - "
+                   "interpreting them as being unequal" :
+                   "Unicode unequal comparison "
+                   "failed to convert both arguments to Unicode - "
+                   "interpreting them as being unequal"
+                   ) < 0)
+        return NULL;
+    result = (op == Py_NE);
+    return PyBool_FromLong(result);
+}
+
 int PyUnicode_Contains(PyObject *container,
 		       PyObject *element)
 {
@@ -6985,11 +7061,14 @@ static PySequenceMethods unicode_as_sequence = {
     PyUnicode_Contains, 		/* sq_contains */
 };
 
+#define HASINDEX(o) PyType_HasFeature((o)->ob_type, Py_TPFLAGS_HAVE_INDEX)
+
 static PyObject*
 unicode_subscript(PyUnicodeObject* self, PyObject* item)
 {
-    if (PyIndex_Check(item)) {
+    PyNumberMethods *nb = item->ob_type->tp_as_number;
-        Py_ssize_t i = PyNumber_AsSsize_t(item, PyExc_IndexError);
+    if (nb != NULL && HASINDEX(item) && nb->nb_index != NULL) {
+        Py_ssize_t i = nb->nb_index(item);
         if (i == -1 && PyErr_Occurred())
             return NULL;
         if (i < 0)
@@ -7859,7 +7938,7 @@ PyTypeObject PyUnicode_Type = {
     0, 					/* tp_print */
     0,				 	/* tp_getattr */
     0, 					/* tp_setattr */
-    (cmpfunc) unicode_compare, 		/* tp_compare */
+    0, 					/* tp_compare */
     unicode_repr, 			/* tp_repr */
     &unicode_as_number, 		/* tp_as_number */
     &unicode_as_sequence, 		/* tp_as_sequence */
@@ -7875,7 +7954,7 @@ PyTypeObject PyUnicode_Type = {
     unicode_doc,			/* tp_doc */
     0,					/* tp_traverse */
     0,					/* tp_clear */
-    0,					/* tp_richcompare */
+    PyUnicode_RichCompare,		/* tp_richcompare */
     0,					/* tp_weaklistoffset */
     0,					/* tp_iter */
     0,					/* tp_iternext */