Re: Accessors in Python (getters and setters)

On 2006-07-15, Gerhard Fiedler <gelists@xxxxxxxxx> 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.
.

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