random.choice([]) should return more intelligible exception

[hoffman]

BPO	964230
Nosy	@rhettinger, @terryjreedy

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2004-06-05.15:51:45.000>
created_at = <Date 2004-06-01.12:39:19.000>
labels = ['library']
title = 'random.choice([]) should return more intelligible exception'
updated_at = <Date 2004-06-05.15:51:45.000>
user = 'https://bugs.python.org/hoffman'

bugs.python.org fields:

activity = <Date 2004-06-05.15:51:45.000>
actor = 'terry.reedy'
assignee = 'none'
closed = True
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2004-06-01.12:39:19.000>
creator = 'hoffman'
dependencies = []
files = []
hgrepos = []
issue_num = 964230
keywords = []
message_count = 7.0
messages = ['20945', '20946', '20947', '20948', '20949', '20950', '20951']
nosy_count = 3.0
nosy_names = ['rhettinger', 'terry.reedy', 'hoffman']
pr_nums = []
priority = 'low'
resolution = None
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue964230'
versions = ['Python 2.4']

[hoffman]

Python 2.3.3 (#1, Mar 31 2004, 11:17:07)
[GCC 3.2.2 20030222 (Red Hat Linux 3.2.2-5)] on linux2
Type "help", "copyright", "credits" or "license" for
more information.
>>> import random
>>> random.choice([])
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "/usr/lib/python2.3/random.py", line 231, in choice
    return seq[int(self.random() * len(seq))]
IndexError: list index out of range

This is simple enough here, but it's harder when it's
at the bottom of a traceback.

I suggest something like ValueError: sequence must not
be empty.

[rhettinger]

Logged In: YES
user_id=80475

-0

While a ValueError would be appropriate, the status quo
doesn't bug me much and changing it could break code if
someone is currently trapping the IndexError.

[hoffman]

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I thought of that after I submitted. :-)

Might it be better to raise an IndexError with a message
similar to "sequence must not be empty" instead? It would
just make debugging that much easier.

[rhettinger]

Logged In: YES
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That's better, but I'm still -0. This function is apt to
be called inside a loop, so it would be a bummer to slow
down everyone's code just to rewrite the error message.

For better or worse, it is the nature of Python tracebacks
to raise RoadWeavingErrors when a DontDrinkAndDriveWarning
would be more to the point.

I recommend closing this, but if you really think it's an
essential life saver, then assign to Tim and see if you can
persuade him.

[hoffman]

Logged In: YES
user_id=987664

You have a point. What about rewriting the line to read:
return seq[int(self.random() * len(seq))] # raises IndexError if
seq is empty

The comment would be a good hint.

[rhettinger]

Logged In: YES
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Okay. Fixed. See Lib/random.py 1.61.

[terryjreedy]

Logged In: YES
user_id=593130

More succinctly:
# empty seq raises IndexError
Commenting source code for traceback clarity has been
suggested before as a way to enhance without breaking.
Works for me

For making repeated choices, however, a generator would be
faster for n much larger than 0. Something like

def chooser(count, seq):
  if type(count) not in (int, long) or count < 0:
    raise ValueError('%s is not a count' % count)
  try:
    seqlen = len(seq)
    if not seqlen: raise ValueError('Cannot choose from 
nothing')
  except TypeError:
    raise ValueError('Seq arg must have length')
  while count:
    yield seq[int(self.random() * seqlen)]
    count -= 1