Move error test to the function that needs it. Improve error message. (GH-30008)

Lib/random.py

@@ -233,10 +233,8 @@ class Random(_random.Random):
                 break
 
     def _randbelow_with_getrandbits(self, n):
-        "Return a random int in the range [0,n).  Returns 0 if n==0."
+        "Return a random int in the range [0,n).  Defined for n > 0."
 
-        if not n:
-            return 0
         getrandbits = self.getrandbits
         k = n.bit_length()  # don't use (n-1) here because n can be 1
         r = getrandbits(k)  # 0 <= r < 2**k
@@ -245,7 +243,7 @@ class Random(_random.Random):
         return r
 
     def _randbelow_without_getrandbits(self, n, maxsize=1<<BPF):
-        """Return a random int in the range [0,n).  Returns 0 if n==0.
+        """Return a random int in the range [0,n).  Defined for n > 0.
 
         The implementation does not use getrandbits, but only random.
         """
@@ -256,8 +254,6 @@ class Random(_random.Random):
                 "enough bits to choose from a population range this large.\n"
                 "To remove the range limitation, add a getrandbits() method.")
             return _floor(random() * n)
-        if n == 0:
-            return 0
         rem = maxsize % n
         limit = (maxsize - rem) / maxsize   # int(limit * maxsize) % n == 0
         r = random()
@@ -338,7 +334,8 @@ class Random(_random.Random):
 
     def choice(self, seq):
         """Choose a random element from a non-empty sequence."""
-        # raises IndexError if seq is empty
+        if not seq:
+            raise IndexError('Cannot choose from an empty sequence')
         return seq[self._randbelow(len(seq))]
 
     def shuffle(self, x):

Lib/test/test_random.py

@@ -816,10 +816,6 @@ class MersenneTwister_TestBasicOps(TestBasicOps, unittest.TestCase):
                 maxsize+1, maxsize=maxsize
             )
         self.gen._randbelow_without_getrandbits(5640, maxsize=maxsize)
-        # issue 33203: test that _randbelow returns zero on
-        # n == 0 also in its getrandbits-independent branch.
-        x = self.gen._randbelow_without_getrandbits(0, maxsize=maxsize)
-        self.assertEqual(x, 0)
 
         # This might be going too far to test a single line, but because of our
         # noble aim of achieving 100% test coverage we need to write a case in