gh-132320: random._randbelow fix

Lib/random.py

@@ -245,7 +245,7 @@ def __init_subclass__(cls, /, **kwargs):
     def _randbelow_with_getrandbits(self, n):
         "Return a random int in the range [0,n).  Defined for n > 0."
 
-        k = n.bit_length()
+        k = (n - 1).bit_length()
         r = self.getrandbits(k)  # 0 <= r < 2**k
         while r >= n:
             r = self.getrandbits(k)

Lib/test/test_random.py

@@ -813,7 +813,7 @@ def test_randrange_uses_getrandbits(self):
         self.gen.seed(1234567)
         # If randrange uses getrandbits, it should pick getrandbits(100)
         # when called with a 100-bits stop argument.
-        self.assertEqual(self.gen.randrange(2**99),
+        self.assertEqual(self.gen.randrange(2**99 + 1),
                          97904845777343510404718956115)
 
     def test_randbelow_logic(self, _log=log, int=int):

Misc/NEWS.d/next/Library/2025-04-09-14-59-52.gh-issue-132320.qa1OoM.rst

@@ -0,0 +1 @@
+20-30% performance improvement when generating integers lower than some power of 2. E.g. :func:`!random.randint(0, 2**32-1)`