execution order of expressions in a list comprehensions

Question

Given the following expressions:

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

A matrix is created, then a list comprehension is executed to create a flat list. The comprehension runs from left to right.

flat = [x for row in matrix for x in row]

Subsequently for each row in the matrix its values are squared. How is this comprehension evaluated?

squared = [[x**2 for x in row] for row in matrix]

let me review that post... thanks for the reference – dcrearer Nov 21 '18 at 19:54 — dcrearer, Nov 21 '18 at 19:54

timgeb · Accepted Answer · 2018-11-21T20:17:06.910

0

The first comprehension is equivalent to:

flat = []
for row in matrix:
    for x in row:
        flat.append(x)

The second comprehension is equivalent to:

squared = []
for row in matrix:
    tmp = []
    for x in row:
        tmp.append(x**2)
    squared.append(tmp)

(Except for creating additional variables in the enclosing scope like x, row, tmp.)

edited Nov 21 '18 at 20:17

answered Nov 21 '18 at 19:53

timgeb

1

thanks for the clear concise sample... – dcrearer Nov 21 '18 at 19:57
@wim right, I overlooked those. The `del`-ing is probably unnecessary to get the point across. – timgeb Nov 21 '18 at 20:17

1 Answers1