assertion failures in warn_explicit() in case of a bad warnings.filters or warnings.defaultaction

[orenmn]

BPO	31416
Nosy	@serhiy-storchaka, @orenmn
PRs	bpo-31416: Fix assertion failures in case of a bad warnings.filters or warnings.defaultaction #3496 [3.6] bpo-31416: Fix assertion failures in case of a bad warnings.fil… #3509

^{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 2017-09-12.06:48:53.490>
created_at = <Date 2017-09-11.07:52:58.758>
labels = ['extension-modules', '3.7', 'type-crash']
title = 'assertion failures in warn_explicit() in case of a bad warnings.filters or warnings.defaultaction'
updated_at = <Date 2017-09-12.06:48:53.489>
user = 'https://github.com/orenmn'

bugs.python.org fields:

activity = <Date 2017-09-12.06:48:53.489>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2017-09-12.06:48:53.490>
closer = 'serhiy.storchaka'
components = ['Extension Modules']
creation = <Date 2017-09-11.07:52:58.758>
creator = 'Oren Milman'
dependencies = []
files = []
hgrepos = []
issue_num = 31416
keywords = ['patch']
message_count = 3.0
messages = ['301867', '301903', '301940']
nosy_count = 2.0
nosy_names = ['serhiy.storchaka', 'Oren Milman']
pr_nums = ['3496', '3509']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue31416'
versions = ['Python 3.7']

[orenmn]

The following code causes an assertion failure in warn_explicit() (in
Python/_warnings.c):

import warnings
warnings.filters = [(None, None, Warning, None, 0)]
warnings.warn_explicit(message='foo', category=Warning, filename='bar',
                       lineno=1)

this is because warn_explicit() assumes that get_filter() returned a string,
and passes the return value (of get_filter()) to _PyUnicode_EqualToASCIIString(),
which asserts it received a string.

In addition, get_filter() might return warnings.defaultaction, and so the
following code also causes an assertion failure in warn_explicit():

import warnings
warnings.defaultaction = None
warnings.filters = []
warnings.warn_explicit(message='foo', category=Warning, filename='bar',
                       lineno=1)

[serhiy-storchaka]

New changeset 9d984fd by Serhiy Storchaka (Oren Milman) in branch 'master':
bpo-31416: Fix assertion failures in case of a bad warnings.filters or warnings.defaultaction. (bpo-3496)
9d984fd

[serhiy-storchaka]

New changeset 9adc87b by Serhiy Storchaka in branch '3.6':
[3.6] bpo-31416: Fix assertion failures in case of a bad warnings.filters or warnings.defaultaction. (GH-3496) (bpo-3509)
9adc87b