def my_function(keyword_arg=None):
"""keyword_arg is a mutable list that persists
across invocations of my_function().
"""
if keyword_arg is None:
keyword_arg = []
keyword_arg.append(1)
return keyword_arg
assert(my_function() == [1])
assert(my_function() == [1]) # False: [1, 1] != [1]