Timed assignments

[magsol]

I'd like to explore the possibility of adding timed assignments in nbgrader. Specifically, within a certain timeframe--

• Students would fetch the assignment, starting an internal timer
• Each assignment would have an "expiration" time attached to it, e.g. 3 hours from fetch
• After this expiration time, nbgrader will no longer allow the students to perform new submissions

This could be used in a "take-home exam" format, or even along the lines of a Google Codejam-style coding competition, where once a student actually begins the assignment they have only a short window in which to finish it before their ability to submit that assignment is disabled.

A quick-and-dirty way of implementing this would be to freeze the exchange directory (even revoke write permissions) for certain users after a certain time frame, but that wouldn't scale too well and would require resetting the permissions for the next assignment. Something more robust would likely require an optional argument for the assignments in nbgrader_config to specify their expiration time limit, and some kind of logging within nbgrader to enforce the time limit when a student attempts to submit.