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bpo-42772: Step argument ignored when stop is None. #24018

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Merged
merged 2 commits into from
Jan 2, 2021
Merged

bpo-42772: Step argument ignored when stop is None. #24018

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ccdac01
bpo-42772: Fix silent failure for step argument when stop is not spe…
rhettinger Dec 31, 2020
46c9782
Add blurb
rhettinger Dec 31, 2020
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8 changes: 7 additions & 1 deletion Lib/random.py
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Original file line number Diff line number Diff line change
Expand Up @@ -96,6 +96,7 @@
SG_MAGICCONST = 1.0 + _log(4.5)
BPF = 53 # Number of bits in a float
RECIP_BPF = 2 ** -BPF
_ONE = 1


class Random(_random.Random):
Expand Down Expand Up @@ -288,7 +289,7 @@ def randbytes(self, n):

## -------------------- integer methods -------------------

def randrange(self, start, stop=None, step=1):
def randrange(self, start, stop=None, step=_ONE):
"""Choose a random item from range(start, stop[, step]).

This fixes the problem with randint() which includes the
Expand All @@ -311,7 +312,12 @@ def randrange(self, start, stop=None, step=1):
_warn('randrange() will raise TypeError in the future',
DeprecationWarning, 2)
raise ValueError("non-integer arg 1 for randrange()")

if stop is None:
# We don't check for "step != 1" because it hasn't been
# type checked and converted to an integer yet.
if step is not _ONE:
raise TypeError('Missing a non-None stop argument')
if istart > 0:
return self._randbelow(istart)
raise ValueError("empty range for randrange()")
Expand Down
8 changes: 8 additions & 0 deletions Lib/test/test_random.py
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Expand Up @@ -562,6 +562,14 @@ def test_randrange_argument_handling(self):
with self.assertRaises(ValueError):
randrange(10, 20, 1.5)

def test_randrange_step(self):
# bpo-42772: When stop is None, the step argument was being ignored.
randrange = self.gen.randrange
with self.assertRaises(TypeError):
randrange(1000, step=100)
with self.assertRaises(TypeError):
randrange(1000, None, step=100)

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))
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2020-12-30-17-16-43.bpo-42772.Xe7WFV.rst
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@@ -0,0 +1,2 @@
randrange() now raises a TypeError when step is specified without a stop
argument. Formerly, it silently ignored the step argument.