Questions about app design - OOP with python classes

[MRAB]

On Mar 2, 4:47 am, Steven D'Aprano <s... at REMOVEME.cybersource.com.au>
wrote:
> On Thu, 01 Mar 2007 20:24:33 -0800, Paul Rubin wrote:
> > Steven D'Aprano <s... at REMOVEME.cybersource.com.au> writes:
> >> But if you used Apps Hungarian, and saw this line of code:
>
> >> if hmmCurrentHeight <= hinCriticalHeight:
>
> >> then you should instantly recognise that there's a problem. Comparing
> >> a height in millimetres to a height in inches is not a good thing to do,
> >> no matter that they're both floats.
>
> > That still sounds like an unreliable manual type system,
>
> It's unreliable in the sense that the coder has to follow the naming
> convention, and must have some bare minimum of sense. If your coders are
> morons, no naming convention will save you. (For that matter, nothing will
> save you.)
>
> > instead of an
> > automatic type system that includes dimension analysis.  You might
> > like this:
>
> >    http://futureboy.homeip.net/frinkdocs/
>
> > There are many Python implementations of dimensioned units as well.  I
> > posted one here a few weeks ago.
>
> Sure, and I love the Unix utility "units", and my HP calculator.
> Dimensioned units are useful.
>
> But dimensioned units are still only part of the story. Joel describes the
> situation the early Word developers found: when you're writing a word
> processor, you are doing a LOT of conversions between screen coordinates
> and window pane coordinates. Both have the same type (a pair of ints),
> both have the same dimensional units (length/pixels) but they are
> semantically different. If you place the character "N" at coordinates 0,0,
> it makes a big difference if the coordinates are relative to the current
> window or relative to the screen.
>
> Apps Hungarian is a heuristic for dealing with semantic differences, not
> data types. It deals with more than just dimensional analysis.
>
Basically what you want are dimensions such as "screen coordinate" and
"window coordinate" as well as "pixel", and also other kinds of
dimensions which combine differently. For example:

The dimension "pixel" works one way and the dimensions "relative to
screen" and "relative to window" work a differnt way.

If:

    p is a point relative to the screen:     integer, <pixel>,
+<relative to screen>

    q is a point relative to the window:     integer, <pixel>,
+<relative to window>

    r is the window relative to the screen:  integer, <pixel>,
+<relative to screen>, -<relative to window>

then:

    p = q + r

    <pixels> = <pixels> + <pixels>

    <relative to screen> = <relative to window> + (<relative to
screen> - <relative to window>)