* Alf P. Steinbach:
* Steven D'Aprano:
On Wed, 04 Nov 2009 08:50:42 +0100, Alf P. Steinbach wrote:
* Gabriel Genellina:
I don't understand either. R1 and R2 have *different* semantics.
Assume that they have the very exact same semantics
Why would we assume that when you have explicitly told us that they
don't?
You stated categorically that they behave differently when you assign
to the attribute/property "top".
Uh, severe reading difficulties ... referring to self in plural ... Hm. :-)
But anyway, in the example description I wrote
"With R1 direct changes of left and top keeps the rectangle's size"
and this is in the context of a discussion of modifying data attributes
directly versus using properties.
Perhaps this makes it more clear: in R1, which has a width/height based
rectangle representation, assigning directly to the top data attribute
/effectively/ moves the rectangle vertically without changing its height, since
the height attribute is unchanged.
But that does not reflect any intended semantics, it's not a requirement; it's
an implementation artifact, a behavior that just results from direct
modification and the choice of a particular rectangle representation.
Real world Python example of that kind of artifact: as discussed in some other
thread here, doing open( ..., 'r+' ) followed by write followed directly by read
will on some implementations/systems produce garbage. Presumably because those
implementations use C "FILE*" to implement the functionality, and implements it
by a fairly direct mapping of calls down to the C level, where this sequence is
in general Undefined Behavior. You might regard it as semantics, and it's quite
real and presumably in a sense well-defined for the particular implementation on
the particular system, but it's not part of any intended semantics, and any who
relies on that behavior is doing it at other's risk.
For the R1 class the indended semantics, the specification that the programmer
was handed down or produced or had in mind, might include just rectangle
construction, checking intersection with other rectangle, and obtaining any of
three pairs of values: left upper corner, right lower corner and width+height.
For example. :-)
Cheers & hth.,
- Alf
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