Testing time sensitive applications in Python

Question

I've written an auction system in Django. I want to write unit tests but the application is time sensitive (e.g. the amount advertisers are charged is a function of how long their ad has been active on a website). What's a good approach for testing this type of application?

Here's one possible solution: a DateFactory class which provides some methods to generate a predictable date in testing and the realtime value in production. Do you have any thoughts on this approach, or have you tried something else in practice?

Answer 1

In the link you provided, the author somewhat rejects the idea of adding additional parameters to your methods for the sake of unit testing, but in some cases I think you can justify this as just an extension of your business logic. In my opinion, it's a form of inversion of control that can make your model more flexible and possibly even more expressive. For example:

def is_expired(self, check_date=None):
    _check_date = check_date or datetime.utcnow()
    return self.create_date + timedelta(days=15) < _check_date

Essentially this allows my unit test to supply its own date/time for the purpose of validating my logic.

The argument in the referenced blog seems to be that this mucks up the API. However, I have encountered situations in which production use cases called for supplanting current date/time with an alternate value. In other words, the inversion of control approach eventually became a necessary part of my application.