While writing my tests I realized, it would be really useful to make write_file() return the path it wrote to. I need the concatenated filenames most of the time, so I'm getting blocks of:

fn = os.path.join(x,y)
write_file(fn, 'contents')

I'd prefer:
fn = write_file((x,y), 'contents')

Any thoughts? IMHO a write_file that concats path but doesn't return it is only useful in the tree-functions.