Launch Python, click on File | New Window Ctrl + N. Generate the five lines of code mentioned in step 3. We include several bugs on purpose.

Click on File and save it.

Comment out the entire program by using a pair of triple quotation marks. Since we need some output, we add a return line as follows:

def bs_call(S,X,T,r,sigma):
    """
    d1 = (lo(S/X)+(r+sigma*sigma/2.)*T)/(sigma*sqrt(T))
    d2 = d1-sigmasqrt(T)
    return SCN(d1)-X*exp(-r*T)*CND(d2)
    """
return (X)

Again, click on Run and then Run from module F5. We will see that the following output appears:

>>>===========RESTART ===============

We test it by using any set of input values as follows:

>>>bs_call(40,40,0.5,0.1,0.2)
40
>>>bs_call...