matrix - Python - Circle patter

I would like to create a circle in zeros matrix by changing specific positions from 0 to 1. Unfortunately, something is wrong in my if statement I cannot figure it out. Thanks for helping me!

img = np.zeros((20,20))
def convert2file(image, newfile):
    img = Image.new('RGB', (len(image), len(image[0])), "black")
    pixels = img.load()

    for i in range(len(image)):
        for j in range (len(image[i])):
            if image[i][j] == 0:
                pixels[j,i] = black
            else:
                pixels[j,i] = white


    save2file(img, newfile)
def gen_circle(image):
    ret = np.copy(image)
    for i in range(len(image)):
        for j in range(len(image[i])):
            if fabs((j - len(image)/2)**2 + (i - len(image)/2)**2  - len(image)/2**2) == 0.5**2:
                ret[i][j] = 1
    return ret

draw_pic(gen_circle(img))

Answer

How about this code:

def gen_circle(image):
    ret = np.copy(image)
    radius = len(image)/2
    radius_sq = radius**2
    for i in range(len(image)):
        for j in range(len(image[i])):
            if (i-radius)**2 + (j-radius)**2 <= radius_sq:
                ret[i][j] = 1
    return ret