Change a mathematical function within certain range

I need to code a mathematical function that will use different value at specific x position. (It would represent light passing through different medium)

I tried to make a very simple code first to see if it would work, and unfortunately I can't seem to properly input it as I get no graph.

def function(x):
    if x < 10:
       a = 1
    if (10.0 <= x <= 20.0):
       a = 0.5
    if x > 20:
       a = -1
    return x*a

x = np.arange (0,30,90)
y = function(x)
plt.plot(x,y,'b')
plt.show()

Thanks