How to get all iterations of a list of lists?

Question

Let's say I have the list:

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

I'm looking to get all of the iterations of the list like this:

[[1], None, None]
[[1,2], None, None]
[[1,2], [3], None]
[[1,2], [3,4], None]
[[1,2], [3,4,5], None]
[[1,2], [3,4,5], [6]]
[[1,2], [3,4,5], [6,7]]
[[1,2], [3,4,5], [6,7,8]]
[[1,2], [3,4,5], [6,7,8,9]]

With the final output being a list of those iterations:

result = [[[1], None, None], [[1,2], None, None], ... , [[1,2], [3,4,5], [6,7,8,9]]]

Answer 1

You can use a list comprehension:

a = [[1,2], [3,4,5], [6,7,8,9]]
new_a = [a[:i]+[b[:k]]+[None]*(len(a)-i-1) for i, b in enumerate(a) for k in range(len(b)+1) if b[:k]]

Output:

[[[1], None, None], 
 [[1, 2], None, None], 
 [[1, 2], [3], None], 
 [[1, 2], [3, 4], None], 
 [[1, 2], [3, 4, 5], None], 
 [[1, 2], [3, 4, 5], [6]], 
 [[1, 2], [3, 4, 5], [6, 7]], 
 [[1, 2], [3, 4, 5], [6, 7, 8]], 
 [[1, 2], [3, 4, 5], [6, 7, 8, 9]]]