barbaros wrote:
Hello everybody,
I am building a code for surface meshes (triangulations for instance).
I need to implement Body objects (bodies can be points, segments,
triangles and so on), then a Mesh will be a collection of bodies,
together with their neighbourhood relations.
I also need OrientedBody objects, which consist in a body
together with a plus or minus sign to describe its orientation.
So, basically an OrientedBody is just a Body with an
integer label stuck on it.
I implemented it in a very crude manner:
------------------------------------------
class Body:
[...]
class OrientedBody:
def __init__ (self,b,orient=1):
# b is an already existing body
assert isinstance(b,Body)
self.base = b
self.orientation = orient
-------------------------------------------
class Body(object) :
...
class OrientedBody (Body):
def __init__(self, orientation = 1) :
Body.__init__(self)
self.orientation = orientation
as noted
But, also.
as a rule of thumb .. if you are using "isinstance" in a class to
determine what class a parameter is ... you have broken the OO contract.
Remember, every class ought to have a well defined internal state and
a well defined interface to its state.
If I write --
class foo (object):
def __init__ :
pass
def some_func (self, val) :
if isinstance (val, "bar") :
....
Then I am either doing something very wrong or very clever (either can
get me in trouble)
In Python it is preferred that I write two functions some_func_a and
some_func_b
e.g.
def some_func_a (self, val = None, class = bar) :
assert(isinstance (class, "bar"), True)
....
def some_func_b (self, val = None, class = baz) :
assert (isinstance (class, "baz"), True)
C++ and Java try to enforce the OO contract by making data and methods
private, protected or public. Which helps -- but leads to some
confusion (what is protected inheritance in C++????) Python exposes all
of its classes internals to everyone -- but that doesn't mean you should
touch them!!
As Larry Wall once wrote, "There is a difference between, 'do not enter
my living room because I asked you not to' and 'do not enter my living
room because I have a shotgun'"
Python adopts the 'do not touch my private parts because I asked you not
to' idiom. (In C++, only friends can touch your privates ... ;-)
So -- be mindful that checking the class of well defined parameters at
anytime is breaking the contract -- you may need to do it -- but it is
more likely that you aren't adhering to good OOD.
Does that make any sense?
Seriously -- I have not had any coffee yet and I am still new at Python.
-- Andrew
My question is: can it be done using inheritance ?
I recall that I need three distinct objects:
the basic (non-oriented) body, the same body with positive
orientation and the same body with negative orientation.
Thank you. Cristian Barbarosie
http://cmaf.fc.ul.pt/~barbaros
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