To calculate the midpoint of a line, use the following formula to calculate the x and y midpoints:

# Middle of line
# midpoint = (xM, yM)
xM = (x1 + x2)/2
yM = (y1 + y2)/2

Then the midpoint coordinates (xM, yM) can be used to draw an ellipse on the line. For example:

ellipse(xM, yM, 10, 10)

Here is a more complete code example, showing this in practice:

x1 = 50
y1 = 50
x2 = 250
y2 = 250

# circle_size
cs = 4

def setup():
    size(300, 300)
    stroke(0)
     
def draw():
    fill(0)
    l1 = line_with_points(x1, y1, x2, y2, cs)
     
class line_with_points(object):
    def __init__(self, x1, y1, x2, y2, cs):
        self.x1 = x1
        self.x2 = x2
        self.y1 = y1
        self.y2 = y2
        self.cs = cs
        
        # Beginning of line
        ellipse(x1, y1, cs, cs)
        # End of line
        ellipse(x2, y2, cs, cs)
        line(x1, y1, x2, y2)
        
        # Middle of line
        xM = (x1 + x2)/2
        yM = (y1 + y2)/2
        fill(255)
        ellipse(xM, yM, cs+5, cs+5)

Which produces: