Python loop (insert items in a list at dermined places)

I have this code:

number = 2
size = 5
list_b = [("b","b","b")]
list_a = [("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"开发者_高级运维),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a")]
for i in range(number):
   list_a.insert(size,list_b)
print list_a

it gives me this:

[('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('b', 'b', 'b'),
('b', 'b', 'b'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a')]

basically, it inserts 2 times the list_b in the position defined by size

I want a loop that repeats itself so that list_b is inserted the number of times defined in number but repeats size times. It difficult to explain, so here is the result that I want:

[('a', 'a', 'a'),
('a', 'a', 'a'), 
('a', 'a', 'a'),
('a', 'a', 'a'), 
('a', 'a', 'a'), 
('b', 'b', 'b'), 
('b', 'b', 'b'), 
('a', 'a', 'a'), 
('a', 'a', 'a'), 
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('b', 'b', 'b'),
('b', 'b', 'b'),
('a', 'a', 'a'),
('a', 'a', 'a'), 
('a', 'a', 'a'), 
('a', 'a', 'a'),
('a', 'a', 'a'),
('b', 'b', 'b'),
('b', 'b', 'b'), 
('a', 'a', 'a'), 
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('b', 'b', 'b'),
('b', 'b', 'b'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('a', 'a', 'a'),
('b', 'b', 'b'),
('b', 'b', 'b'),...and so on]

EDIT

and if I had this:

 list_a = [a, ] * 15
 list_b = [b,]
 s = 5
 n = 2

I want to obtain this:

 [b,b,a,a,a,a,a,b,b,b,b,a,a,a,a,a,b,b,b,b,a,a,a,a,a,b,b]

since this is an example and list_a, s and n will vary, how can I do this in one or two loops?

Thanks,

Favolas

For the sake of the argument, I'll call the ('a', 'a', 'a') => a and ('b', 'b', 'b') => b.

number=2
size=5
list_a=["a"]*20
list_b=["b"]

workfor=len(list_a)+(len(list_a)/size)*number*len(list_b)
i=0
while i<workfor:
    i+=size
    for times in range(number):
        for elem in list_b:
            list_a.insert(i,elem)
        i+=len(list_b)

print list_a

Results in =>

 ['a', 'a', 'a', 'a', 'a', 'b', 'b', 'a', 'a', 'a', 'a', 'a', 'b', 'b', 'a', 'a', 'a', 'a', 'a', 'b', 'b', 'a', 'a', 'a', 'a', 'a', 'b', 'b']

#!/usr/bin/python

number = 2
size = 5
list_b = [("b","b","b")]
list_a = [("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a"),("a","a","a")]

if __name__ == '__main__':
    insertion_count = len(list_a) / size
    for j in xrange(insertion_count):
        # compute insertion position
        pos = (j+1)*size + j * number
        for i in range(number):
            list_a.insert(pos,list_b)
    print list_a

from itertools import chain, izip, repeat

list_a = [('a', 'a', 'a')] * 15
list_b = [('b', 'b', 'b')]

a5b2s = [iter(list_a)] * 5 + [repeat(*list_b)] * 2
list_a[:] = chain.from_iterable(izip(*a5b2s))

>>> s,n=5,2
>>> a=[1,]*17
>>> b=2
>>> for i in range(len(a)//s*s,0,-s):
    for j in range(n):
        a.insert(i,b)

>>> a
[1, 1, 1, 1, 1, 2, 2, 1, 1, 1, 1, 1, 2, 2, 1, 1, 1, 1, 1, 2, 2, 1, 1]