Python - slope and collinear

def slope(x1, y1, x2, y2):
    '''calculate a in formula y=a*x+b'''

    dx = (x2 - x1)
    dy = (y2 - y1)

    a = 0
    b = 0

    if dx != 0:
        a = dy/dx # slope (tangens)

    #b = y1 - a*x1 # or b = y2 - a*x2
    #print(b)

    return a

slope1 = slope(0, 1, 1, 2) # first line
slope2 = slope(0, 0, 1, 1) # second line

print('Slopes to y=0:', slope1, slope2)
print('Collinear:', slope1 == slope2)