Question

(Edit: randrange is just random.randrange, I didn’t write my own RNG)

I’m trying to create a list of instances of a class I defined. Here’s the entire class (by request):

from random import randrange

class Poly:
    points = [0] * 8
    fill = 'red'
    alpha = 1.0

    def __init__(self, width=100, height=100):
        for i in range(0, 8, 2):
            self.points[i] = randrange(width)
            self.points[i+1] = randrange(height)
        self.alpha = random()
        return

Seems to work fine:

>>> for i in range(5):
        Poly().points

[28, 64, 93, 26, 15, 31, 44, 50]
[24, 14, 47, 14, 35, 17, 63, 62]
[99, 28, 90, 29, 56, 59, 57, 33]
[62, 56, 48, 28, 40, 73, 70, 99]
[99, 32, 27, 99, 42, 57, 86, 12]

But if I try to create a list of these objects, I get separate instances (different memory addresses) but they all have the same random values:

>>> p = []
>>> for i in range(5):
        p.append(Poly())

>>> p
[<gen_image.Poly instance at 0x02D773C8>, <gen_image.Poly instance at 0x02D77FD0>, <gen_image.Poly instance at 0x0321D030>, <gen_image.Poly instance at 0x02D51E40>, <gen_image.Poly instance at 0x02D51DA0>]

>>> for poly in p:
        print poly.points

[75, 18, 5, 76, 6, 64, 95, 54]
[75, 18, 5, 76, 6, 64, 95, 54]
[75, 18, 5, 76, 6, 64, 95, 54]
[75, 18, 5, 76, 6, 64, 95, 54]
[75, 18, 5, 76, 6, 64, 95, 54]

What’s going on here? And what’s the right way to do what I’m trying to do?

Answer 1

Move the creation of the array into the __init__ method.

You’re working with a shared array among all objects.

The reason the first shows different is that you print the contents of that array before you construct a new Poly object and thus trample over the array contents. If you had kept them around and inspected them later they would all appear to have the same contents as the last one you generated.

Oh, and try not to simplify code when posting questions. Always post complete, but short, programs that reproduce the problem.

Here’s a short, but complete, program that demonstrates the problem you’re having:

from random import randrange
class Poly:
    points = [0]*8

    def __init__(self, width=100, height=100):
        for i in range(0, 8, 2):
            self.points[i] = randrange(width)
            self.points[i+1] = randrange(height)
        return

p1 = Poly()
print "p1:", p1.points
p2 = Poly()
print "p2:", p2.points
print "p1:", p1.points

Sample output:

[C:\Temp] test.py
p1: [19, 5, 1, 46, 93, 18, 18, 57]
p2: [92, 71, 42, 84, 54, 29, 27, 71]
p1: [92, 71, 42, 84, 54, 29, 27, 71]

Notice how p1 changed.

The fixed code could be as simple as:

from random import randrange
class Poly:
    def __init__(self, width=100, height=100):
        self.points = [0]*8
        for i in range(0, 8, 2):
            self.points[i] = randrange(width)
            self.points[i+1] = randrange(height)
        return

although I prefer the append variant that @Doug posted here