random.choice on empty sequence should hide previous exception [patch]

[then0rTh]

BPO	29055
Nosy	@rhettinger, @mdickinson, @MojoVampire
Files	random_choice_errmsg.patch: patch for random.py

^{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 = 'https://github.com/rhettinger'
closed_at = <Date 2016-12-27.09:09:33.828>
created_at = <Date 2016-12-23.15:56:57.938>
labels = ['3.7', 'type-bug', 'library']
title = 'random.choice on empty sequence should hide previous exception [patch]'
updated_at = <Date 2016-12-27.09:09:33.786>
user = 'https://bugs.python.org/then0rTh'

bugs.python.org fields:

activity = <Date 2016-12-27.09:09:33.786>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2016-12-27.09:09:33.828>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2016-12-23.15:56:57.938>
creator = 'then0rTh'
dependencies = []
files = ['46007']
hgrepos = []
issue_num = 29055
keywords = ['patch']
message_count = 4.0
messages = ['283884', '283886', '284074', '284075']
nosy_count = 5.0
nosy_names = ['rhettinger', 'mark.dickinson', 'python-dev', 'josh.r', 'then0rTh']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue29055'
versions = ['Python 3.6', 'Python 3.7']

[then0rTh]

Passing empty sequence to random.choice function leads to:

Traceback (most recent call last):
  ...
ValueError: number of bits must be greater than zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  ...
IndexError: Cannot choose from an empty sequence

• the ValueError doesn't add any useful information, only bloats stderr
• the "During handling" line indicates that something went wrong inside random.py

This patch uses raise x from None to hide the ValueError, resulting in much cleaner output.

-Tested on Python 3.7.0a0

[MojoVampire]

Seems reasonable to me.

[python-dev]

New changeset 0001ae913759 by Raymond Hettinger in branch '3.6':
Issue bpo-29055: Suppress upstream exception for random.choice()
https://hg.python.org/cpython/rev/0001ae913759

[rhettinger]

Thanks for the patch.

Please submit a contributor agreement for future work. We would like to have it for all patches (though this one was so tiny it isn't strictly required).