Most third-party Python libraries throw custom exceptions. Many of these exceptions have their own dependencies and side effects. Consider, for example, the following situation: class ThirdPartyException(BaseException): def __init__(self): print("I do something arcane and expensive upon construction.") print("Maybe I have a bunch of arguments that can't be None, too.") def state(self) -> bool: # In real life, this could be True or False return True Let's say, moreover, that I absolutely have to handle this exception, and to do it, I need to look at the exception's state. If I want to write tests to examine the behavior when this exception is handled, I must have the ability to create a ThirdPartyException. But I may not even be able to figure out how, let alone how to do it cheaply. If this weren't an Exception and I wanted to write tests, I would immediately reach for MagicMock. But I cannot figure out how to use MagicMock with an exception. How do I test the error handling cases in the following code, ideally using py.test? def error_causing_thing(): raise ThirdPartyException() def handle_error_conditionally(): try: error_causing_thing() exception ThirdPartyException as e: if state: return "Some non-error value" else: return "A different non-error value" Continue reading...
