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).

Python/bltinmodule.c

@@ -338,6 +338,24 @@ PyDoc_STRVAR(filter_doc,
 "function is None, return the items that are true.  If sequence is a tuple\n"
 "or string, return the same type, else return a list.");
 
+static PyObject *
+builtin_format(PyObject *self, PyObject *args)
+{
+	PyObject *value;
+	PyObject *format_spec = NULL;
+
+	if (!PyArg_ParseTuple(args, "O|O:format", &value, &format_spec))
+		return NULL;
+
+	return PyObject_Format(value, format_spec);
+}
+
+PyDoc_STRVAR(format_doc,
+"format(value[, format_spec]) -> string\n\
+\n\
+Returns value.__format__(format_spec)\n\
+format_spec defaults to \"\"");
+
 static PyObject *
 builtin_chr(PyObject *self, PyObject *args)
 {
@@ -2359,6 +2377,7 @@ static PyMethodDef builtin_methods[] = {
  	{"eval",	builtin_eval,       METH_VARARGS, eval_doc},
  	{"execfile",	builtin_execfile,   METH_VARARGS, execfile_doc},
  	{"filter",	builtin_filter,     METH_VARARGS, filter_doc},
+ 	{"format",	builtin_format,     METH_VARARGS, format_doc},
  	{"getattr",	builtin_getattr,    METH_VARARGS, getattr_doc},
  	{"globals",	(PyCFunction)builtin_globals,    METH_NOARGS, globals_doc},
  	{"hasattr",	builtin_hasattr,    METH_VARARGS, hasattr_doc},

Python/formatter_string.c

@@ -0,0 +1,15 @@
+/***********************************************************************/
+/* Implements the string (as opposed to unicode) version of the
+   built-in formatters for string, int, float.  That is, the versions
+   of int.__float__, etc., that take and return string objects */
+
+#include "Python.h"
+#include "formatter_string.h"
+
+#include "../Objects/stringlib/stringdefs.h"
+
+#define FORMAT_STRING string__format__
+#define FORMAT_LONG   string_long__format__
+#define FORMAT_INT    string_int__format__
+#define FORMAT_FLOAT  string_float__format__
+#include "../Objects/stringlib/formatter.h"

Python/formatter_unicode.c

@@ -0,0 +1,13 @@
+/* Implements the unicode (as opposed to string) version of the
+   built-in formatter for unicode.  That is, unicode.__format__(). */
+
+#include "Python.h"
+#include "formatter_unicode.h"
+
+#include "../Objects/stringlib/unicodedefs.h"
+
+#define FORMAT_STRING unicode__format__
+/* don't define FORMAT_LONG and FORMAT_FLOAT, since we can live
+   with only the string versions of those.  The builtin format()
+   will convert them to unicode. */
+#include "../Objects/stringlib/formatter.h"