Nested comprehensions in Python

Comprehensions over two dimensions: grids and matrices.

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

flat = [cell for row in grid for cell in row]
doubled = [[cell * 2 for cell in row] for row in grid]
diagonal = [grid[i][i] for i in range(len(grid))]

coords = [(r, c) for r in range(3) for c in range(3) if r != c]

table = {r: [c * r for c in range(4)] for r in range(3)}

How it works

1. Two for clauses flatten a grid of rows into cells.
2. A comprehension inside a comprehension rebuilds rows.
3. The pattern extends to dict comprehensions of lists.

Keywords and builtins used here

• for
• if
• len
• range

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Step 3 of 4 in Comprehensions, step 3 of 53 in Pythonic Python.

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