extmod/modurandom: Support an argument of bits=0 to getrandbits.

This was changed in CPython 3.9; see https://bugs.python.org/issue40282. Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

extmod/modurandom.c

@@ -87,9 +87,12 @@ STATIC uint32_t yasmarang_randbelow(uint32_t n) {
 
 STATIC mp_obj_t mod_urandom_getrandbits(mp_obj_t num_in) {
  int n = mp_obj_get_int(num_in);
- if (n > 32 || n == 0) {
+ if (n > 32 || n < 0) {
  mp_raise_ValueError(MP_ERROR_TEXT("bits must be 32 or less"));
  }
+ if (n == 0) {
+ return MP_OBJ_NEW_SMALL_INT(0);
+ }
  uint32_t mask = ~0;
  // Beware of C undefined behavior when shifting by >= than bit size
  mask >>= (32 - n);

tests/extmod/urandom_basic.py

@@ -22,8 +22,11 @@
 random.seed(1)
 print(random.getrandbits(16) == r)
 
-# check that it throws an error for zero bits
+# check that zero bits works
+print(random.getrandbits(0))
+
+# check that it throws an error for negative bits
 try:
- random.getrandbits(0)
+ random.getrandbits(-1)
 except ValueError:
  print("ValueError")

tests/extmod/urandom_basic.py.exp

@@ -0,0 +1,4 @@
+True
+True
+0
+ValueError