py/objstr: Add support for the :_b/o/x specifier in str.format.

This groups non-decimal values by fours, such as bbb_bbbb_bbbb.  It also
supports `{:_d}` to use underscore for decimal numbers (grouped in threes).

Use of incorrect ":,b" is not diagnosed.

Thanks to @dpgeorge for the suggestion to reduce code size.

Signed-off-by: Jeff Epler <jepler@gmail.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/py/mpz.c b/py/mpz.c
index 084aebda9..471bd1598 100644
--- a/py/mpz.c
+++ b/py/mpz.c
@@ -1672,6 +1672,8 @@ size_t mpz_as_str_inpl(const mpz_t *i, unsigned int base, const char *prefix, ch
 
     size_t ilen = i->len;
 
+    int n_comma = (base == 10) ? 3 : 4;
+
     char *s = str;
     if (ilen == 0) {
         if (prefix) {
@@ -1717,7 +1719,7 @@ size_t mpz_as_str_inpl(const mpz_t *i, unsigned int base, const char *prefix, ch
                 break;
             }
         }
-        if (!done && comma && (s - last_comma) == 3) {
+        if (!done && comma && (s - last_comma) == n_comma) {
             *s++ = comma;
             last_comma = s;
         }