List append not working

Question

So I have something similar to this:

listPoints=[]
for x in range(100):
    for y in range(10):
        point[0]=x
        point[1]=y
        point[2] = num_cluster
        listPoints.append(point)
print listPoints

And when I print listPoints the list only contains the values of the last iteration..what I'm doing wrong? I don't see it

Answer 1

When you append point, you're not adding a copy of the point array -- you're adding a reference to it. So, when you do listPoints.append(point), you're essentially adding the exact same reference to the exact same thing each time. Consequently, when you change point, it appears as if every element in listPoints also changes.

You can fix the problem by creating a list instead:

listPoints=[]
for x in range(100):
    for y in range(10):
        point = [x, y, num_cluster]
        listPoints.append(point)
print listPoints

...or by copying it, if you absolutely must:

import copy

listPoints=[]
for x in range(100):
    for y in range(10):
        new_point = copy.deepcopy(point)
        new_point[0]=x
        new_point[1]=y
        new_point[2] = num_cluster
        listPoints.append(new_point)
print listPoints