Index: Objects/unicodeobject.c
===================================================================
--- Objects/unicodeobject.c	(revision 62896)
+++ Objects/unicodeobject.c	(working copy)
@@ -8276,6 +8276,25 @@
 static PyObject *
 unicode_mod(PyObject *v, PyObject *w)
 {
+       /* Keep track of our recursion level.  We don't want to print out
+          warnings when calling into PyErr_WarnEx, so use 'level' as
+          a counter for how we've many times we've called into this
+          function.  This is thread-safe (because we already have the GIL),
+          but it will skip some output if the GIL is released inside the
+          warning machinery.  The code in warnings.py does not use %
+          formatting, so this could only be a problem with user code.
+          Making 'level' into a thread-local variable doesn't seem worth
+          the hassle or runtime expense. */
+       static int level = 0;
+
+       if (level == 0) {
+               int err;
+               ++level;
+               err = PyErr_WarnEx(PyExc_PendingDeprecationWarning,
+                        "% formatting will be deprecated, "
+                        "use .format() instead", 1);
+               --level;
+       }
        if (!PyUnicode_Check(v)) {
                Py_INCREF(Py_NotImplemented);
                return Py_NotImplemented;
Index: Misc/NEWS
===================================================================
--- Misc/NEWS	(revision 62896)
+++ Misc/NEWS	(working copy)
@@ -12,6 +12,9 @@
 Core and Builtins
 -----------------
 
+- Issue #2772: Added PendingDeprecationWarning for % formatting.  Use
+  str.format() instead.
+
 Extension Modules
 -----------------
 
Index: Lib/warnings.py
===================================================================
--- Lib/warnings.py	(revision 62896)
+++ Lib/warnings.py	(working copy)
@@ -22,11 +22,12 @@
 
 def formatwarning(message, category, filename, lineno, line=None):
     """Function to format a warning the standard way."""
-    s =  "%s:%s: %s: %s\n" % (filename, lineno, category.__name__, message)
+    s =  "{0}:{1}: {2}: {3}\n".format(filename, lineno,
+                                      category.__name__, message)
     line = linecache.getline(filename, lineno) if line is None else line
     if line:
         line = line.strip()
-        s += "  %s\n" % line
+        s += "  {0}\n".format(line)
     return s
 
 def filterwarnings(action, message="", category=Warning, module="", lineno=0,
@@ -36,7 +37,7 @@
     Use assertions to check that all arguments have the right type."""
     import re
     assert action in ("error", "ignore", "always", "default", "module",
-                      "once"), "invalid action: %r" % (action,)
+                      "once"), "invalid action: {0!r}".format(action)
     assert isinstance(message, str), "message must be a string"
     assert isinstance(category, type), "category must be a class"
     assert issubclass(category, Warning), "category must be a Warning subclass"
@@ -56,7 +57,7 @@
     A simple filter matches all modules and messages.
     """
     assert action in ("error", "ignore", "always", "default", "module",
-                      "once"), "invalid action: %r" % (action,)
+                      "once"), "invalid action: {0!r}".format(action)
     assert isinstance(lineno, int) and lineno >= 0, \
            "lineno must be an int >= 0"
     item = (action, None, category, None, lineno)
@@ -86,7 +87,7 @@
     import re
     parts = arg.split(':')
     if len(parts) > 5:
-        raise _OptionError("too many fields (max 5): %r" % (arg,))
+        raise _OptionError("too many fields (max 5): {0!r}".format(arg))
     while len(parts) < 5:
         parts.append('')
     action, message, category, module, lineno = [s.strip()
@@ -103,7 +104,7 @@
             if lineno < 0:
                 raise ValueError
         except (ValueError, OverflowError):
-            raise _OptionError("invalid lineno %r" % (lineno,))
+            raise _OptionError("invalid lineno {0!r}".format(lineno))
     else:
         lineno = 0
     filterwarnings(action, message, category, module, lineno)
@@ -116,7 +117,7 @@
     for a in ('default', 'always', 'ignore', 'module', 'once', 'error'):
         if a.startswith(action):
             return a
-    raise _OptionError("invalid action: %r" % (action,))
+    raise _OptionError("invalid action: {0!r}".format(action))
 
 # Helper for _setoption()
 def _getcategory(category):
@@ -127,7 +128,8 @@
         try:
             cat = eval(category)
         except NameError:
-            raise _OptionError("unknown warning category: %r" % (category,))
+            raise _OptionError("unknown warning category: {0!r}".format(
+                category))
     else:
         i = category.rfind(".")
         module = category[:i]
@@ -135,13 +137,14 @@
         try:
             m = __import__(module, None, None, [klass])
         except ImportError:
-            raise _OptionError("invalid module name: %r" % (module,))
+            raise _OptionError("invalid module name: {0!r}".format(module))
         try:
             cat = getattr(m, klass)
         except AttributeError:
-            raise _OptionError("unknown warning category: %r" % (category,))
+            raise _OptionError("unknown warning category: {0!r}".format(
+                category))
     if not issubclass(cat, Warning):
-        raise _OptionError("invalid warning category: %r" % (category,))
+        raise _OptionError("invalid warning category: {0!r}".format(category))
     return cat
 
 
@@ -245,8 +248,8 @@
     else:
         # Unrecognized actions are errors
         raise RuntimeError(
-              "Unrecognized action (%r) in warnings.filters:\n %s" %
-              (action, item))
+              "Unrecognized action ({0!r}) "
+              "in warnings.filters:\n {1}".format(action, item))
     if not hasattr(showwarning, "__call__"):
         raise TypeError("warnings.showwarning() must be set to a "
                         "function or method")