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)