I have some code that checks that a file is treated differently if some time has passed. In other words, it reacts if the modification time is more than it was before. The code uses os.stat(filename)[stat.ST_MTIME] to do this. The challenge was to mock os.stat so that I can pretend some time has passed without having to wait. Here was my first solution which made running many tests sloooow:

Here's one of many things I've learnt today at PyCon. Inspired by code that Grig Gheorghiu showed in his slides on automated testing, you can monkey patch a standard library that your application is using in your unit tests to, in my case, mock a remote service without having to run a server. I've done lots of monkey-patching in Zope but then I've only been monkey patching individual methods or attributes of imported classes. This is very similar to that. Here's what my application does: