List of Objects

[DillonCo]

On Thursday 19 April 2007, datamonkey.ryan at gmail.com wrote:
> Howdy, a (possibly) quick question for anyone willing to listen.
> I have a question regarding lists and Classes; I have a class called
> "gazelle" with several attributes (color, position, etc.) and I need
> to create a herd of them. I want to simulate motion of individual
> gazelles, but I don't want to have to go through and manually update
> the position for every gazelle (there could be upwards of 50). I was
> planning to create an array of these gazelle classes, and I was going
> to iterate through it to adjust the position of each gazelle. That's
> how I'd do it in C, anyway. However, Python doesn't support pointers
> and I'm not quite sure how to go about this. Any help you can provide
> would be greatly appreciated.
> Thanks a lot!

Actually, Python _only_ supports pointers:  they're the names of objects.  So 
for example, if I write x = Gazelle(...), then I create the name "x" that 
points to an instance of Gazelle.  The storage for the instance is 
managed 'magically' by Python.  If I were then to say "y = x", I'd also have 
a name "y" that points to the same instance.

It's also worth noting that everything, including ints, strings and lists are 
objects as well.  Because of this, a pointer to an object is the only storage 
class in python.  Therefore, a name can point to any object of any type.

As a result, an "array" in Python, which is commonly a list, is simply a list 
of pointers.  They can point to strings, ints, other lists or anything.  And 
because they store pointers, they can actually include themself!

To demonstrate:
>>> a=[1,'a',[1,2,3]]
>>> for i in a: print i
1
a
[1, 2, 3]
>>> a.append(a)
>>> for i in a: print i
1
a
[1, 2, 3]
[1, 'a', [1, 2, 3], [...]]


Python's clever enough to not print out the circular reference.

Finally, it's worth pointing out that in a language like this, where there are 
no arbitrary pointers (as there are in C), the pointer-to-object is called a 
reference.  I just used "pointer" because you did ;).