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)

New changeset 9d984fd2b097c8c29479d1c3eb740995fe1ccb0d by Serhiy Storchaka (Oren Milman) in branch 'master':
bpo-31416: Fix assertion failures in case of a bad warnings.filters or warnings.defaultaction. (#3496)
https://github.com/python/cpython/commit/9d984fd2b097c8c29479d1c3eb740995fe1ccb0d

New changeset 9adc87b0f82e5169c5f44739f89212a86013d1c4 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) (#3509)
https://github.com/python/cpython/commit/9adc87b0f82e5169c5f44739f89212a86013d1c4