Programming Python, 3rd Edition

Reloading Callback Handlers Dynamically

Our next GUI-programming technique is all about changing a GUI while it is running—the ultimate in customization. The Python reload function lets you dynamically change and reload a program’s modules without stopping the program. For instance, you can bring up a text editor window to change the source code of selected parts of a system while it is running and see those changes show up immediately after reloading the changed module.

This is a powerful feature, especially for developing programs that take a long time to restart. Programs that connect to databases or network servers, initialize large objects, or travel through a long series of steps to retrigger a callback are prime candidates for reload. It can shave substantial time from the development cycle.

The catch for GUIs, though, is that because callback handlers are registered as object references rather than module and object names, reloads of callback handler functions are ineffective after the callback has been registered. The Python reload operation works by changing a module object’s contents in place. Because Tkinter stores a pointer to the registered handler object directly, though, it is oblivious to any reloads of the module that the handler came from. That is, Tkinter will still reference a module’s old objects even after the module is reloaded and changed.

This is a subtle thing, but you really only need to remember that you must do something special to reload callback ...