Generating our own module p4f

We could combine many small Python progams as one program, such as p4f.py. For instance, the preceding Python program called bs_call() function is included. Such a collection of programs offers several benefits. First, when we use the bs_call() function, we don't have to type those five lines. To save space, we only show a few functions included in p4f.py. For brevity, we remove all comments included for each function. Those comments are designed to help future users when issuing the help() function, such as help(bs_call()):

def bs_call(S,X,T,rf,sigma):
    from scipy import log,exp,sqrt,stats
    d1=(log(S/X)+(rf+sigma*sigma/2.)*T)/(sigma*sqrt(T))
    d2 = d1-sigma*sqrt(T)
    return S*stats.norm.cdf(d1)-X*exp(-rf*T)*stats.norm.cdf(d2)

def binomial_grid(n):
    import networkx as nx 
    import matplotlib.pyplot as plt 
    G=nx.Graph() 
    for i in range(0,n+1):     
        for j in range(1,i+2):         
            if i<n:             
            ...