When working in an interactive session, Python only loads a module from its source file once, the first time it is imported.

Spyder’s User Module Reloader (UMR) automatically reloads modules right in your existing IPython consoles whenever they are modified and re-imported. With the UMR enabled, you can test changes to your code without restarting the kernel.

UMR is enabled by default, and it will provide you with a red Reloaded modules: message in the console listing the files it has refreshed when it is activated. If desired, you can turn it on or off, and prevent specific modules from being reloaded, under Preferences ‣ Python interpreter ‣ User Module Reloader (UMR).