On 2006-07-15, Gerhard Fiedler <[EMAIL PROTECTED]> wrote: > On 2006-07-15 06:55:14, mystilleef wrote: > >> In very well designed systems, the state of an object should only be >> changed by the object. > > IMO that's not quite true. Ultimately, the state always gets changed by > something else (user interaction, physical events); very few objects are > completely self-contained in their behavior. > > In most systems (and you possibly have written some of them) are objects > whose state gets changed by other objects -- possibly through the > intermediation of setter methods that do nothing else but set the state. > There's no conceptual difference between directly setting the state or > calling a setter function that does nothing else but directly setting the > state -- except for one unnecessary level of indirection in the latter.
The conceptual difference is that a function call is more like an "event", a variable is more like a "process". An object that provides a setter is a process that is prepared to engage in this "set event". An object that exposes a variable is a process that interacts with this variable which the user is also invited to interact with concurrently. So with a setter in the API, conceptually, there are two processes sharing the set event: the object itself, and the process that's calling the setter. With an exposed variable, there are three: the object, the variable in between, and the calling process. Restricting yourself to setters and getters is a bit like saying we build a machine that only has touch-keys and lights on the outside. Shared variables are more like a machine with levers and dials you set, and bits that pop up, like the numbers on mechanical cash-registers. They have "state" on the outside, not just on the inside. Such designs can be less tolerant of unsympathetic input-- think how easy it is to jam up an old-fashioned typewriter if you just press a few keys at the same time. There isn't any practical difference, as you say, if all the setter does is set. But it might easily do a few other subtle things, in particular wait for a good moment to actually effect the change. -- http://mail.python.org/mailman/listinfo/python-list
