On 8/6/2008 7:23 AM, Jonny Morrill wrote:
Thank you both for your help!!
@Matt:
WOW! I never new that you could create an iterable object from only the
__len__ and operator[] functions!! I must have missed that when I was
searching through the python documentation for iterator objects. It is not
clear to me which is quicker, the only difference seems to be that when you
use __iter__, you can return a copy of the object so that any modifications
will not affect the original object. Thanks again!
Actually, __iter__ shouldn't modify anything. The simplest way to
implement __iter__ in Python is to write a generator function which
yields the elements.
__len__ and __getitem__ predate __iter__ (and iterators, and generators)
by far: it is the oldest and easier way to iterate over an object in a
for loop. If you think of it, it is obvious that it is possible to do a
for-loop over an object that represents a collection and already exposes
__len__ and __getitem__.
To be specific, you don't even need __len__: it's just an optimization
exploited internally by Python. It's sufficient to have __getitem__ and
raises IndexError whenever the sequence is finished.
@Giovanni:
That seems to be exactly what I was after. I have a few questions about the
implementation though.
Since I do use the next() function as part of my C++ API i might just keep
my iteration the way it is currently. I use it to iterate similar to the
following code:
Item* Object::next()
{
if(this->currentIndex < this->length) {
return item[this->currentIndex];
} else {
return NULL;
}
}
// In Code
Object* object = new Object;
Item* item;
while(item = object->next()) {
// do something with item
}
I wouldn't call this a good design: your iteration actually does modify
the object. So you can iterate onto the object only once (or you need a
resetIteration function); the object is not thread-safe; etc.
Anyway, surely it's not what Python expects from the iteration function.
Oh!! So the %MethodCode doesn't require a partner C++ function? You can
specify python specific functions inside it??
Yes. Given the code you showed me, probably it's sufficient something
like this in the sip file (pseudo-code, untest):
class Object
{
Item* __getitem__(int);
%MethodCode
if (a0 >= sipCpp->length)
{
PyErr_SetNone(PyExc_IndexError);
sipIsErr = 1;
}
else
{
Item *item = sipCpp->items[a0];
sipResult = sipConvertToInstance([...]);
}
%End
}
I'll let you find out the details of how to write the code. Refer to the
SIP documentation.
--
Giovanni Bajo
Develer S.r.l.
http://www.develer.com
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