Thank you Anvillistas, we will add a notice to the documentation to let people know that you can only access the filesystem from the Full Python environments.
The Basic Python environments are PyPy Sandboxes. That means they politely ask us for permission before making system calls, so we can keep users completely secure from one another on the same machine. It’s the magic tech that lets us provide hosted Python back-ends for free! (And we sponsored the creation of the Python 3 version.) Of course, this all means that Basic back-ends don’t have access to the filesystem. Here’s a 5-minute talk by a very enthusiastic Meredydd about how it all works.