Python3 & Tkinter, list.sort list of objects made of StringVar
You didn't quite get the syntax right for the lambda. You need an argument and a colon. Try:
list.sort(Action._registry, key=lambda item: item.__dict__['date'].get())Additional style tip: it is idiomatic to call sort as a method of the object you're sorting, rather than literally doing list.sort.
Action._registry.sort(key=lambda item: item.__dict__['date'].get())