diff --git a/Doc/library/random.rst b/Doc/library/random.rst index 51242cb0e95..9964af46b2e 100644 --- a/Doc/library/random.rst +++ b/Doc/library/random.rst @@ -111,6 +111,9 @@ Bookkeeping functions as an optional part of the API. When available, :meth:`getrandbits` enables :meth:`randrange` to handle arbitrarily large ranges. + .. versionchanged:: 3.9 + This method now accepts zero for *k*. + .. function:: randbytes(n) diff --git a/Lib/random.py b/Lib/random.py index 82345fab921..3243938282b 100644 --- a/Lib/random.py +++ b/Lib/random.py @@ -261,6 +261,8 @@ class Random(_random.Random): def _randbelow_with_getrandbits(self, n): "Return a random int in the range [0,n). Raises ValueError if n==0." + if not n: + raise ValueError("Boundary cannot be zero") 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 @@ -733,8 +735,8 @@ class SystemRandom(Random): def getrandbits(self, k): """getrandbits(k) -> x. Generates an int with k random bits.""" - if k <= 0: - raise ValueError('number of bits must be greater than zero') + if k < 0: + raise ValueError('number of bits must be non-negative') numbytes = (k + 7) // 8 # bits / 8 and rounded up x = int.from_bytes(_urandom(numbytes), 'big') return x >> (numbytes * 8 - k) # trim excess bits diff --git a/Lib/test/test_random.py b/Lib/test/test_random.py index f709e52ecb8..efac36a63b9 100644 --- a/Lib/test/test_random.py +++ b/Lib/test/test_random.py @@ -263,6 +263,31 @@ class TestBasicOps: self.assertEqual(x1, x2) self.assertEqual(y1, y2) + def test_getrandbits(self): + # Verify ranges + for k in range(1, 1000): + self.assertTrue(0 <= self.gen.getrandbits(k) < 2**k) + self.assertEqual(self.gen.getrandbits(0), 0) + + # Verify all bits active + getbits = self.gen.getrandbits + for span in [1, 2, 3, 4, 31, 32, 32, 52, 53, 54, 119, 127, 128, 129]: + all_bits = 2**span-1 + cum = 0 + cpl_cum = 0 + for i in range(100): + v = getbits(span) + cum |= v + cpl_cum |= all_bits ^ v + self.assertEqual(cum, all_bits) + self.assertEqual(cpl_cum, all_bits) + + # Verify argument checking + self.assertRaises(TypeError, self.gen.getrandbits) + self.assertRaises(TypeError, self.gen.getrandbits, 1, 2) + self.assertRaises(ValueError, self.gen.getrandbits, -1) + self.assertRaises(TypeError, self.gen.getrandbits, 10.1) + def test_pickling(self): for proto in range(pickle.HIGHEST_PROTOCOL + 1): state = pickle.dumps(self.gen, proto) @@ -390,26 +415,6 @@ class SystemRandom_TestBasicOps(TestBasicOps, unittest.TestCase): raises(0, 42, 0) raises(0, 42, 3.14159) - def test_genrandbits(self): - # Verify ranges - for k in range(1, 1000): - self.assertTrue(0 <= self.gen.getrandbits(k) < 2**k) - - # Verify all bits active - getbits = self.gen.getrandbits - for span in [1, 2, 3, 4, 31, 32, 32, 52, 53, 54, 119, 127, 128, 129]: - cum = 0 - for i in range(100): - cum |= getbits(span) - self.assertEqual(cum, 2**span-1) - - # Verify argument checking - self.assertRaises(TypeError, self.gen.getrandbits) - self.assertRaises(TypeError, self.gen.getrandbits, 1, 2) - self.assertRaises(ValueError, self.gen.getrandbits, 0) - self.assertRaises(ValueError, self.gen.getrandbits, -1) - self.assertRaises(TypeError, self.gen.getrandbits, 10.1) - def test_randbelow_logic(self, _log=log, int=int): # check bitcount transition points: 2**i and 2**(i+1)-1 # show that: k = int(1.001 + _log(n, 2)) @@ -629,34 +634,18 @@ class MersenneTwister_TestBasicOps(TestBasicOps, unittest.TestCase): self.assertEqual(set(range(start,stop)), set([self.gen.randrange(start,stop) for i in range(100)])) - def test_genrandbits(self): + def test_getrandbits(self): + super().test_getrandbits() + # Verify cross-platform repeatability self.gen.seed(1234567) self.assertEqual(self.gen.getrandbits(100), 97904845777343510404718956115) - # Verify ranges - for k in range(1, 1000): - self.assertTrue(0 <= self.gen.getrandbits(k) < 2**k) - - # Verify all bits active - getbits = self.gen.getrandbits - for span in [1, 2, 3, 4, 31, 32, 32, 52, 53, 54, 119, 127, 128, 129]: - cum = 0 - for i in range(100): - cum |= getbits(span) - self.assertEqual(cum, 2**span-1) - - # Verify argument checking - self.assertRaises(TypeError, self.gen.getrandbits) - self.assertRaises(TypeError, self.gen.getrandbits, 'a') - self.assertRaises(TypeError, self.gen.getrandbits, 1, 2) - self.assertRaises(ValueError, self.gen.getrandbits, 0) - self.assertRaises(ValueError, self.gen.getrandbits, -1) def test_randrange_uses_getrandbits(self): # Verify use of getrandbits by randrange # Use same seed as in the cross-platform repeatability test - # in test_genrandbits above. + # in test_getrandbits above. self.gen.seed(1234567) # If randrange uses getrandbits, it should pick getrandbits(100) # when called with a 100-bits stop argument. diff --git a/Misc/NEWS.d/next/Library/2020-04-15-10-23-52.bpo-40282.rIYJmu.rst b/Misc/NEWS.d/next/Library/2020-04-15-10-23-52.bpo-40282.rIYJmu.rst new file mode 100644 index 00000000000..699282a7fb5 --- /dev/null +++ b/Misc/NEWS.d/next/Library/2020-04-15-10-23-52.bpo-40282.rIYJmu.rst @@ -0,0 +1 @@ +Allow ``random.getrandbits(0)`` to succeed and to return 0. diff --git a/Modules/_randommodule.c b/Modules/_randommodule.c index 51c084288fc..64e44e3bc9f 100644 --- a/Modules/_randommodule.c +++ b/Modules/_randommodule.c @@ -474,12 +474,15 @@ _random_Random_getrandbits_impl(RandomObject *self, int k) uint32_t *wordarray; PyObject *result; - if (k <= 0) { + if (k < 0) { PyErr_SetString(PyExc_ValueError, - "number of bits must be greater than zero"); + "number of bits must be non-negative"); return NULL; } + if (k == 0) + return PyLong_FromLong(0); + if (k <= 32) /* Fast path */ return PyLong_FromUnsignedLong(genrand_uint32(self) >> (32 - k));