Addresses issue 2802: 'n' formatting for integers.

Adds 'n' as a format specifier for integers, to mirror the same
specifier which is already available for floats.  'n' is the same as
'd', but inserts the current locale-specific thousands grouping.

I added this as a stringlib function, but it's only used by str type,
not unicode.  This is because of an implementation detail in
unicode.format(), which does its own str->unicode conversion.  But the
unicode version will be needed in 3.0, and it may be needed by other
code eventually in 2.6 (maybe decimal?), so I left it as a stringlib
implementation.  As long as the unicode version isn't instantiated,
there's no overhead for this.

Objects/stringlib/localeutil.h

@@ -0,0 +1,121 @@
+/* stringlib: locale related helpers implementation */
+
+#ifndef STRINGLIB_LOCALEUTIL_H
+#define STRINGLIB_LOCALEUTIL_H
+
+#include <locale.h>
+
+/**
+ * _Py_InsertThousandsGrouping:
+ * @buffer: A pointer to the start of a string.
+ * @len: The length of the string.
+ * @plast: A pointer to the end of of the digits in the string.  This
+ *         may be before the end of the string (if the string contains
+ *         decimals, for example).
+ * @buf_size: The maximum size of the buffer pointed to by buffer.
+ * @count: If non-NULL, points to a variable that will receive the
+ *         number of characters we need to insert (and no formatting
+ *         will actually occur).
+ * @append_zero_char: If non-zero, put a trailing zero at the end of
+ *         of the resulting string, if and only if we modified the
+ *         string.
+ *
+ * Inserts thousand grouping characters (as defined in the current
+ *  locale) into the string between buffer and plast.  If count is
+ *  non-NULL, don't do any formatting, just count the number of
+ *  characters to insert.  This is used by the caller to appropriately
+ *  resize the buffer, if needed.
+ *
+ * Return value: 0 on error, else 1.  Note that no error can occur if
+ *  count is non-NULL.
+ *
+ * This name won't be used, the includer of this file should define
+ *  it to be the actual function name, based on unicode or string.
+ **/
+int
+_Py_InsertThousandsGrouping(STRINGLIB_CHAR *buffer,
+			    Py_ssize_t len,
+			    STRINGLIB_CHAR *plast,
+			    Py_ssize_t buf_size,
+			    Py_ssize_t *count,
+			    int append_zero_char)
+{
+	struct lconv *locale_data = localeconv();
+	const char *grouping = locale_data->grouping;
+	const char *thousands_sep = locale_data->thousands_sep;
+	Py_ssize_t thousands_sep_len = strlen(thousands_sep);
+	STRINGLIB_CHAR *pend = buffer + len; /* current end of buffer */
+	STRINGLIB_CHAR *pmax = buffer + buf_size;       /* max of buffer */
+	char current_grouping;
+
+	/* Initialize the character count, if we're just counting. */
+	if (count)
+		*count = 0;
+
+	/* Starting at plast and working right-to-left, keep track of
+	   what grouping needs to be added and insert that. */
+	current_grouping = *grouping++;
+
+	/* If the first character is 0, perform no grouping at all. */
+	if (current_grouping == 0)
+		return 1;
+
+	while (plast - buffer > current_grouping) {
+		/* Always leave buffer and pend valid at the end of this
+		   loop, since we might leave with a return statement. */
+
+		plast -= current_grouping;
+		if (count) {
+			/* We're only counting, not touching the memory. */
+			*count += thousands_sep_len;
+		}
+		else {
+			/* Do the formatting. */
+
+			/* Is there room to insert thousands_sep_len chars? */
+			if (pmax - pend < thousands_sep_len)
+				/* No room. */
+				return 0;
+
+			/* Move the rest of the string down. */
+			memmove(plast + thousands_sep_len,
+				plast,
+				(pend - plast) * sizeof(STRINGLIB_CHAR));
+			/* Copy the thousands_sep chars into the buffer. */
+#if STRINGLIB_IS_UNICODE
+			/* Convert from the char's of the thousands_sep from
+			   the locale into unicode. */
+			{
+				Py_ssize_t i;
+				for (i = 0; i < thousands_sep_len; ++i)
+					plast[i] = thousands_sep[i];
+			}
+#else
+			/* No conversion, just memcpy the thousands_sep. */
+			memcpy(plast, thousands_sep, thousands_sep_len);
+#endif
+		}
+
+		/* Adjust end pointer. */
+		pend += thousands_sep_len;
+
+		/* Move to the next grouping character, unless we're
+		   repeating (which is designated by a grouping of 0). */
+		if (*grouping != 0) {
+			current_grouping = *grouping++;
+			if (current_grouping == CHAR_MAX)
+				/* We're done. */
+				break;
+		}
+	}
+	if (append_zero_char) {
+		/* Append a zero character to mark the end of the string,
+		   if there's room. */
+		if (pend - plast < 1)
+			/* No room, error. */
+			return 0;
+		*pend = 0;
+	}
+	return 1;
+}
+#endif /* STRINGLIB_LOCALEUTIL_H */