[Edu-sig] Uniform Access Principle confusion

Kirby Urner urnerk at qwest.net
Fri Oct 14 01:34:34 CEST 2005

> But in my (perhaps overly strict) way of OO thinking the unit of measure
> is an attribute of the angle, not of the Triangle.

I think of the unit of measure more as an attribute of the viewer, so if
this were MVC-style OO, we'd probably keep the model in some internal unit,
perhaps integer, while translating to time/size units (e.g. ergs) per some
request into the model by a view, as mediated by the controller.

For example, with the Triangle class, we might just define __slots__ for 3
xyz tuples (per your point re __slots__, yes, for economy in memory) -- and
that's it.  A viewer class might glom on and suck down a reference to a
triangle object, and from self's three slots spit back all angle and edge
measures, in whatever language or units.  A French Imperial bot could spit
back in French.

> Is there a way to a accomplish a "change of state" of the angle
> "attribute", other by sending it a parameter?
> Art

Well, you could do as follows:

 >>> mytri = Triangle()
 >>> mytri.C.degrees
 >>> mytri.C.radians

In this case, the angles are thought of as "sub objects" each with their own
sets of attributes, in this case both radian and angle measure.  The
subobjects might also support actions, e.g. mytri.C.break() or
mytri.C.colorize('Red') -- note verbs.

Just mytri.Cd and mytri.Cr would likely be less confusing (degrees and
radians respectively -- we maybe don't know if these are implemented as
properties unless we have access to the source code).

Note that many programmers visualize a GUI on an object, when coding with
it, i.e. they'll translate the published API, all spelled out in formal
syntax, in terms of slider bars, popup menus, other widgets (it's like the
cockpit of an airplane, once the objects get complicated and plentiful
enough).  That's like a customized "programmer specific" view *into* a
model.  The model itself contains to views.

I saw some examples of this the other night at PORPIG.  Dylan projected some
Zope 3 off his Linux laptop, showing a simple 'hello world' object being
defined in terms of MVC.  A .py file defines M.  V is handled by interfaces
(links to another template XML: .zcml).  These go in product subdirectories.
C (the Controller) is Zope itself in this case i.e. py classes + interfaces
+ Zope = MVC.


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