[workthrowaway12]

i miss Andrew... haven't seen him talk lately. but anyways, not sure i understand the mock file system slide -- it's still calling os.Open?

[y4mi]

Check the date in the URL. Might explain your confusion

[selljamhere]

The slide demonstrates a concrete implementation of the interface, showing how it can be used to communicate with the real file system. The mock implementation is left as an exercise for the reader, as you and I would likely have different requirements.