Backport of PEP 3101, Advanced String Formatting, from py3k.

Highlights:
 - Adding PyObject_Format.
 - Adding string.Format class.
 - Adding __format__ for str, unicode, int, long, float, datetime.
 - Adding builtin format.
 - Adding ''.format and u''.format.
 - str/unicode fixups for formatters.

The files in Objects/stringlib that implement PEP 3101 (stringdefs.h,
unicodedefs.h, formatter.h, string_format.h) are identical in trunk
and py3k.  Any changes from here on should be made to trunk, and
changes will propogate to py3k).

Lib/test/test_builtin.py

@@ -2012,6 +2012,101 @@ class TestSorted(unittest.TestCase):
         data = 'The quick Brown fox Jumped over The lazy Dog'.split()
         self.assertRaises(TypeError, sorted, data, None, lambda x,y: 0)
 
+    def test_format(self):
+        # Test the basic machinery of the format() builtin.  Don't test
+        #  the specifics of the various formatters
+        self.assertEqual(format(3, ''), '3')
+
+        # Returns some classes to use for various tests.  There's
+        #  an old-style version, and a new-style version
+        def classes_new():
+            class A(object):
+                def __init__(self, x):
+                    self.x = x
+                def __format__(self, format_spec):
+                    return str(self.x) + format_spec
+            class DerivedFromA(A):
+                pass
+
+            class Simple(object): pass
+            class DerivedFromSimple(Simple):
+                def __init__(self, x):
+                    self.x = x
+                def __format__(self, format_spec):
+                    return str(self.x) + format_spec
+            class DerivedFromSimple2(DerivedFromSimple): pass
+            return A, DerivedFromA, DerivedFromSimple, DerivedFromSimple2
+
+        # In 3.0, classes_classic has the same meaning as classes_new
+        def classes_classic():
+            class A:
+                def __init__(self, x):
+                    self.x = x
+                def __format__(self, format_spec):
+                    return str(self.x) + format_spec
+            class DerivedFromA(A):
+                pass
+
+            class Simple: pass
+            class DerivedFromSimple(Simple):
+                def __init__(self, x):
+                    self.x = x
+                def __format__(self, format_spec):
+                    return str(self.x) + format_spec
+            class DerivedFromSimple2(DerivedFromSimple): pass
+            return A, DerivedFromA, DerivedFromSimple, DerivedFromSimple2
+
+        def class_test(A, DerivedFromA, DerivedFromSimple, DerivedFromSimple2):
+            self.assertEqual(format(A(3), 'spec'), '3spec')
+            self.assertEqual(format(DerivedFromA(4), 'spec'), '4spec')
+            self.assertEqual(format(DerivedFromSimple(5), 'abc'), '5abc')
+            self.assertEqual(format(DerivedFromSimple2(10), 'abcdef'),
+                             '10abcdef')
+
+        class_test(*classes_new())
+        class_test(*classes_classic())
+
+        def empty_format_spec(value):
+            # test that:
+            #  format(x, '') == str(x)
+            #  format(x) == str(x)
+            self.assertEqual(format(value, ""), str(value))
+            self.assertEqual(format(value), str(value))
+
+        # for builtin types, format(x, "") == str(x)
+        empty_format_spec(17**13)
+        empty_format_spec(1.0)
+        empty_format_spec(3.1415e104)
+        empty_format_spec(-3.1415e104)
+        empty_format_spec(3.1415e-104)
+        empty_format_spec(-3.1415e-104)
+        empty_format_spec(object)
+        empty_format_spec(None)
+
+        # TypeError because self.__format__ returns the wrong type
+        class BadFormatResult:
+            def __format__(self, format_spec):
+                return 1.0
+        self.assertRaises(TypeError, format, BadFormatResult(), "")
+
+        # TypeError because format_spec is not unicode or str
+        self.assertRaises(TypeError, format, object(), 4)
+        self.assertRaises(TypeError, format, object(), object())
+
+        # tests for object.__format__ really belong elsewhere, but
+        #  there's no good place to put them
+        x = object().__format__('')
+        self.assert_(x.startswith('<object object at'))
+
+        # first argument to object.__format__ must be string
+        self.assertRaises(TypeError, object().__format__, 3)
+        self.assertRaises(TypeError, object().__format__, object())
+        self.assertRaises(TypeError, object().__format__, None)
+
+        # make sure we can take a subclass of str as a format spec
+        class DerivedFromStr(str): pass
+        self.assertEqual(format(0, DerivedFromStr('10')), '         0')
+
 def test_main(verbose=None):
     test_classes = (BuiltinTest, TestSorted)