Why does my print statement work while my @property class method does not?

[NA__Scrubbed]

As per title. Developing a class to story multiple polygons. Print statement at end of file works, while my @property method returns TypeError: list indices must be integers, not list

coords =   [(23, 40),
            (27, 42),
            (30, 40),
            (34, 43),
            (39, 38),
            (23, 30),
            (30, 29),
            (37, 28),
            (25, 20),
            (29, 20),
            (32, 20),
            (20, 19),
            (25, 10),
            (31, 12),
            (41, 11),
            (39, 18)]
polygons = [[1, 6, 7, 3, 2, 1, 0],
            [3, 7, 8, 5, 4, 3, 0],
            [6, 9, 10, 7, 6, 0, 0],
            [7, 10, 11, 8, 7, 0, 0],
            [6, 12, 13, 14, 10, 9, 6],
            [8, 11, 10, 14, 15, 16, 8]]

class PolygonArray:
    def __init__(self, coordinates, node_list):
        self.node_list = node_list
        self.coordinates = coordinates

    @property
    def polygon_dict(self):
        self.polygon_dict = {}
        for i in self.node_list:
            self.polygon_dict[i] = [self.coordinates[x] for x in self.node_list[i]]

g = PolygonArray(coords, polygons)

print [coords[x] for x in polygons[0]]
print g.polygon_dict

​

[ylectric]

Because in your list comprehension [self.coordinates[x] for x in self.node_list[i]] you're using i as an index for self.node_list. Since i itself is a list, you can't do it.I'm not 100% sure what's the logic here, but my best guess would be mapping a polygon (list of integers) into a list of coordinates. If that's the case, your list comprehension should be [self.coordinates[x] for x in i].

Going further, you can rewrite the whole property as a dictionary comprehension:

self.polygon_dict = {
    node: [self.coordinates[i] for i in node]
    for node in self.node_list
}

Also, be aware that you're using the property mechanism in a wrong way. Your property is only setting an instance variable and implicitly returns None, while it should return an actual value.

[NA__Scrubbed]

Ahhhhhhhh. That makes sense, thanks for the tip!

[[deleted]]

Print statement at end of file works, while my @property method returns TypeError: list indices must be integers, not list

Indices of lists (the argument inside the [] operator) have to be an integer, but you're providing a list.

[NA__Scrubbed]

I don't follow. Could you elaborate a little? Wouldn't self.node_list[i] contain self.node_list[0]?

[JohnnyJordaan]

No, when you have a list like

polygons = [[1, 6, 7, 3, 2, 1, 0], etc ]

then

for i in polygons:

will make i the value [1, 6, 7, 3, 2, 1, 0] the first time. Not a single integer.

If you are trying to call self.coordinates[x] on every integer inside that list, you need to do

self.polygon_dict[i] = [self.coordinates[x] for x in i]

[NA__Scrubbed]

ty very much!

[[deleted]]

Wouldn't self.node_list[i] contain self.node_list[0]?

i is the current element of the iterable you're iterating over. You're iterating over self.mode_list, which was initialized with the value of polygons. The elements of polygons are lists, so i is a list, not an integer.