The following code causes an assertion failure (in case there exists a
compressed zip file named 'foo.zip' with a file called 'bar.py' in it):

import zlib
import zipimport
def bad_decompress(*args):
    return None

zlib.decompress = bad_decompress
zipimport.zipimporter('foo.zip').get_source('bar')


This is because get_data() (in Modules/zipimport.c) assumes that
zlib.decompress() returned a bytes object, and returns it.
zipimport_zipimporter_get_source_impl() assumes that get_data() returned a
bytes object, and passes it to PyBytes_AS_STRING(), which asserts it is a bytes
object.