[New-bugs-announce] [issue33203] random.choice: raise IndexError on empty sequence even when not using getrandbits internally

[Wolfgang Maier]

New submission from Wolfgang Maier <wolfgang.maier at biologie.uni-freiburg.de>:

from https://docs.python.org/3/library/random.html#random.choice:

Return a random element from the non-empty sequence seq. If seq is empty, raises IndexError.

Indeed:
>>> import random
>>> random.choice([])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/wolma/cpython/random.py", line 259, in choice
    raise IndexError('Cannot choose from an empty sequence') from None
IndexError: Cannot choose from an empty sequence

but when not using getrandbits internally:
>>> class MyRandom(random.Random):
...     def random(self):
...         return super().random()
... 
>>> my_random=MyRandom()
>>> my_random.choice([])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/wolma/cpython/random.py", line 257, in choice
    i = self._randbelow(len(seq))
  File "/home/wolma/cpython/random.py", line 245, in _randbelow
    rem = maxsize % n
ZeroDivisionError: integer division or modulo by zero

This is because the ValueError that random.choice tries to catch gets raised only in the getrandbits-dependent branch of Random._randbelow, but not in the branch using only Random.random (even though Random._randbelow suggests uniform behaviour.

----------
components: Library (Lib)
messages: 314787
nosy: rhettinger, serhiy.storchaka, wolma
priority: normal
severity: normal
status: open
title: random.choice: raise IndexError on empty sequence even when not using getrandbits internally
type: behavior
versions: Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33203>
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