Factorial in a functional style

factorial.py

def factorial(i):                                                               
    if i > 1:                                                                   
        return factorial(i - 1) * i                                             
    elif i == 1:                                                                
        return 1                                                                
    else:                                                                       
        raise ValueError('Cannot take factorial of %d' % i)                     
                                                                                

filter_odd.py

def filter_odd(l):                                                              
    if l == []:                                                                 
        return []                                                               
    elif l[0] % 2:                                                              
        return [l[0]] + filter_odd(l[1:])                                       
    else:                                                                       
        return [] + filter_odd(l[1:])                                           
    # The way it should be...                                                   
    #return [i for i in l if i % 2]                                             

sum.py

def sum(l):                                                                     
    if len(l) >= 1:                                                             
        return l[0] + sum(l[1:])                                                
    else:                                                                       
        return 0