Why is my for loop only capturing y values of 699 instead of values ranging from 0 to 699?

Question

I am programming an arcade game in pygame. I want every black pixel to be a walking surface for the player. I am trying to get_at every black pixel on screen and add it to a dict. Then, I want to compare the x and y coords of the player to the dict entries. When I print the dict Pixels, it only prints y=699 and x values range from 0 to 1250. How can I make walkable black surfaces by adding every black Pixel to a dict for comparison with player coords?

Pixels = {}

for x1 in range(0, 1250):
    for y2 in range(0, 700):
        xy12 = disp.get_at((x1, y2))
        if xy12[0] <= 3 and xy12[1] <= 3 and xy12[2] <= 3: #the <= just makes it so rgb values of 3 are close enough to black that it is acceptable...
             Pixels[x1] = y2

def pglc():
    global groundlim
    if playerx in Pixels:
        if playery == Pixels[playerx]:
            groundlim = playery

print Pixels

Answer 1

This line is your problem

Pixels[x1] = y2

In the inner loop you are overwriting the value in each step, so the last value you see is 699 (but it had all the values from 0 to 698 before when the condition was met). You could make two dimensions:

Pixels[x1] = list()

And then in the inner loop:

Pixels[x1].append(y2)

In total:

Pixels = {}

for x1 in range(0, 1250):
    Pixels[x1] = list()
    for y2 in range(0, 700):
        xy12 = disp.get_at((x1, y2))
        if xy12[0] <= 3 and xy12[1] <= 3 and xy12[2] <= 3:
             Pixels[x1].append(y2)

Or - if you're into dict comprehensions (plus it uses all(...)):

Pixels = {x1:
              [y2 
               for y2 in range(0, 700) 
               for xy12 in [disp.get_at((x1, y2))]
               if (all(cond <= 3 for cond in xy12[0:2]))]
          for x1 in range(0, 1250)}