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/string.py

@@ -527,3 +527,115 @@ try:
     letters = lowercase + uppercase
 except ImportError:
     pass                                          # Use the original versions
+
+########################################################################
+# the Formatter class
+# see PEP 3101 for details and purpose of this class
+
+# The hard parts are reused from the C implementation.  They're
+# exposed here via the sys module.  sys was chosen because it's always
+# available and doesn't have to be dynamically loaded.
+
+# The overall parser is implemented in str._formatter_parser.
+# The field name parser is implemented in str._formatter_field_name_split
+
+class Formatter(object):
+    def format(self, format_string, *args, **kwargs):
+        return self.vformat(format_string, args, kwargs)
+
+    def vformat(self, format_string, args, kwargs):
+        used_args = set()
+        result = self._vformat(format_string, args, kwargs, used_args, 2)
+        self.check_unused_args(used_args, args, kwargs)
+        return result
+
+    def _vformat(self, format_string, args, kwargs, used_args, recursion_depth):
+        if recursion_depth < 0:
+            raise ValueError('Max string recursion exceeded')
+        result = []
+        for literal_text, field_name, format_spec, conversion in \
+                self.parse(format_string):
+
+            # output the literal text
+            if literal_text:
+                result.append(literal_text)
+
+            # if there's a field, output it
+            if field_name is not None:
+                # this is some markup, find the object and do
+                #  the formatting
+
+                # given the field_name, find the object it references
+                #  and the argument it came from
+                obj, arg_used = self.get_field(field_name, args, kwargs)
+                used_args.add(arg_used)
+
+                # do any conversion on the resulting object
+                obj = self.convert_field(obj, conversion)
+
+                # expand the format spec, if needed
+                format_spec = self._vformat(format_spec, args, kwargs,
+                                            used_args, recursion_depth-1)
+
+                # format the object and append to the result
+                result.append(self.format_field(obj, format_spec))
+
+        return ''.join(result)
+
+
+    def get_value(self, key, args, kwargs):
+        if isinstance(key, (int, long)):
+            return args[key]
+        else:
+            return kwargs[key]
+
+
+    def check_unused_args(self, used_args, args, kwargs):
+        pass
+
+
+    def format_field(self, value, format_spec):
+        return format(value, format_spec)
+
+
+    def convert_field(self, value, conversion):
+        # do any conversion on the resulting object
+        if conversion == 'r':
+            return repr(value)
+        elif conversion == 's':
+            return str(value)
+        elif conversion is None:
+            return value
+        raise ValueError("Unknown converion specifier {0!s}".format(conversion))
+
+
+    # returns an iterable that contains tuples of the form:
+    # (literal_text, field_name, format_spec, conversion)
+    # literal_text can be zero length
+    # field_name can be None, in which case there's no
+    #  object to format and output
+    # if field_name is not None, it is looked up, formatted
+    #  with format_spec and conversion and then used
+    def parse(self, format_string):
+        return format_string._formatter_parser()
+
+
+    # given a field_name, find the object it references.
+    #  field_name:   the field being looked up, e.g. "0.name"
+    #                 or "lookup[3]"
+    #  used_args:    a set of which args have been used
+    #  args, kwargs: as passed in to vformat
+    def get_field(self, field_name, args, kwargs):
+        first, rest = field_name._formatter_field_name_split()
+
+        obj = self.get_value(first, args, kwargs)
+
+        # loop through the rest of the field_name, doing
+        #  getattr or getitem as needed
+        for is_attr, i in rest:
+            if is_attr:
+                obj = getattr(obj, i)
+            else:
+                obj = obj[i]
+
+        return obj, first