Angle between 2 points

Calculate the angle in radians, between the horizontal through the 1st point and a line through the two points

def angle(p0,p1):
    dx=float(p1.x)-float(p0.x)
    dy=float(p1.y)-float(p0.y)
    if dx==0:
        if dy==0:
            return 0.0
        elif dy<0:
            return math.atan(float('-inf'))
        else:
            return math.atan(float('inf'))
    return math.atan(dy/dx)