manipulate list of list using random and restrictions

I have two lists a and b.

a=[[3, 1], [3, 2, 3], [5, 1, 4, 8], [3, 5], [10, 1, 3, 6, 7],[2,1,2]]
b=[[4], [3], [9], [5], [21],[5]]

List of list a is interpreted as follows.

• a[0]=[3,1]. This means I want 3 items, where item 1 must be included.
• a[-1]=[2,1,2] I want 2 items, where item 1 and item 2 must
  be included.
• a[2] = [5,1,4,8] I want 5 items, where item 1, item 4 and item 8 must be included.

List of list b is the available list of item from which list a can take from.

• b[0]=[4] this means item 1, item 2, item 3 and item 4 is available. This is linked
  to a[0]
• b[-1]=[5] item 1, item 2, item 3, item 4 and item 5 is available. This is linked to a[-1]
• b[2]=[9] this means i have 9 items is available. This is linked to a[2]

I’m trying to create a new list of list c such that it produces the following result.

For a[0]=[3,1] and b[0]=[4] it should produce the following result [1,2,4]

For a[-1]=[2,1,2] and b[-1]=[5] it should produce [1,2]

For a[2] = [5,1,4,8] and b[2]=[9] should produce [1,4,8,3,9]

What I have tried so far:

import random

a=[[3, 1], [3, 2, 3], [5, 1, 4, 8], [3, 5], [10, 1, 3, 6, 7],[2,1,2]]

b=[[4], [3], [9], [5], [21],[5]]

c=[sorted(random.sample(range(1, j[0]+1), int(i[0]))) for i,j in zip(a,b)]

print(c) # produces [[1, 3, 4], [1, 2, 3], [1, 3, 4, 6, 9], [1, 4, 5], [2, 5, 6, 7, 13, 15, 16, 19, 20, 21], [3, 4]]

As you can see, list c does not take into account those restrictions that I need.

Desired output will look something like this.

c= [[1,2,4],[2,3,1],[1,4,8,3,9],[5,1,3],[1,3,6,7,2,5,9,10,20,21],[1,2]]

>Solution :

You can run a loop over the two lists and construct a remaining_items list to randomly sample from –

 c = []
 for constraints, items in zip(a, b):
    num_items, *required_items = constraints
    pick_list = list(_ for _ in range(1, items[0] + 1) if _ not in required_items)
    num_items_to_pick = num_items - len(required_items)
    remaining_items = random.sample(pick_list, num_items_to_pick)
    print(required_items + remaining_items)
    c.append(required_items + remaining_items)

Output

[[1, 2, 3],
 [2, 3, 1],
 [1, 4, 8, 3, 6],
 [5, 3, 1],
 [1, 3, 6, 7, 18, 2, 14, 21, 19, 15],
 [1, 2]]