Storing a marker in module objects which can be used to validate the linecache is a good idea. timestamp isn't appropriate because of modules loaded from zips or dynamic generation. I'd suggest we make it something opaque - we get source code by asking the loader for it anyway - if we make all source code loading go through the loader, then file based loaders can use a timestamp, zip based ones can shortcircuit or use a reference to the containing zip's file metadata or whatever.