Resources represent data which was loaded and can be reloaded from an original source. Resource derivates must implement LastModified and update() to allow for precise caching and on-demand reload.
Return a datetime.datetime object referring to the timestamp of last modification of the resource stored in this object (not the one represented by this object).
It MUST refer to the date of last modification of the source at the time it was loaded or the timestamp of modification by any program code, which is not neccessarily the same as the current timestamp of last modification of the original source.
The Resource class provides basic means to make your update thread safe: The update() method will only be called with the _updatelock held, if you don’t overwrite this method.
Note that this method will only be called by the framework itself; If you call update() on your own, you will not be safeguarded.