# Python – Understand and extend using lists

Understand and extend using lists… here is a solution to the problem.

## Understand and extend using lists

I have this

``````rows = self.rows()
aaa = []
for r in range(0, 9, 3):
bbb = []
for c in range(0, 9, 3):
ccc = []
for s in range(3):
ccc.extend(rows[r+s][c:c+3])
bbb.append(ccc)
aaa.append(bbb)
``````

It returns this

``````[
[
[5, 0, 0, 0, 6, 0, 0, 2, 9],
[3, 8, 0, 4, 9, 2, 0, 0, 6],
[0, 6, 2, 1, 0, 0, 3, 0, 4]
],
[
[0, 7, 6, 0, 0, 8, 0, 4, 0],
[0, 4, 0, 2, 0, 5, 0, 3, 1],
[0, 3, 1, 0, 4, 9, 0, 0, 0]
],
[
[4, 0, 0, 6, 0, 3, 0, 0, 1],
[0, 0, 0, 7, 0, 0, 0, 0, 0],
[0, 5, 3, 2, 0, 8, 0, 0, 6]
]
]
``````

That’s right.
`rows` Just a list of 9 other nested lists, each with exactly 9 integers, ranging from 0 to 9.

When I try to use list understanding

``````[[rows[r+s][c:c+3] for s in range(3) for c in range(0, 9, 3)] for r in range(0, 9, 3)]
``````

I understand

``````[
[
[5, 0, 0],
[3, 8, 0],
[0, 6, 2],
[0, 6, 0],
[4, 9, 2],
[1, 0, 0],
[0, 2, 9],
[0, 0, 6],
[3, 0, 4]
],
[
[0, 7, 6],
[0, 4, 0],
[0, 3, 1],
[0, 0, 8],
[2, 0, 5],
[0, 4, 9],
[0, 4, 0],
[0, 3, 1],
[0, 0, 0]
],
[
[4, 0, 0],
[0, 0, 0],
[0, 5, 3],
[6, 0, 3],
[7, 0, 0],
[2, 0, 8],
[0, 0, 1],
[0, 0, 0],
[0, 0, 6]
]
]
``````

Apparently I did something wrong, but what can’t I see? I checked other SO issues and they hinted at somehow structuring the LC to prevent the innermost list from splitting into 9 separate lists, but so far it hasn’t happened.

### Solution

Try:

``````import itertools

[[list(itertools.chain(*[rows[r+s][c:c+3] for s in range(3)])) for c in range(0, 9, 3)] for r in range(0, 9, 3)]
``````