testing -- what to do for testing code with behaviour dependant upon which files exist?

[Terry Reedy]

"Brian van den Broek" <bvande at po-box.mcgill.ca> wrote in message 
news:424F00D5.9020205 at po-box.mcgill.ca...
> Hi all,
>
> I'm just starting to employ unit testing (I'm using doctest), and I am
> uncertain how to handle writing tests where the behaviour being tested is 
> dependant on whether certain file paths point to actual files.

I would think about writing a dummy 'file' function whose return or raising 
depends on the filename passed in.  Then your test would be independent of 
the actual environment.  For something more elaborate, I would google 
Google's Python newgroup archive for 'dummy filesystem' for past threads on 
the topic.

TJR