Comprehension

List comprehension

>>> [i for i in range(5)]
[0, 1, 2, 3, 4]

Tip

There is a shortcut for constructing a list in Construct Iterable.

Nested list comprehension

>>> [(i,j) for i in range(5) for j in range(i) ]
[(1, 0),
 (2, 0),
 (2, 1),
 (3, 0),
 (3, 1),
 (3, 2),
 (4, 0),
 (4, 1),
 (4, 2),
 (4, 3)]

>>> [j for i in range(5) for j in range(i)]
[0, 0, 1, 0, 1, 2, 0, 1, 2, 3]

Key point to remember here (which took me a long time to realize) is that the order of the for statements is to be written in the order you’d expect them to be written in a standard for loop, from the outside inwards.

Dict comprehension

>>> ls = [1,2,3]
>>> d = {i: i+1 for i in ls}
>>> d
{1: 2, 2: 3, 3: 4}

Set comprehension

>>> ls = [1, 2, 2, 3]
>>> s = {i for i in ls}
>>> s
{1, 2, 3}

Generator comprehension

>>> ls = [1, 2, 2, 3]
>>> s = (i for i in ls)
>>> list(s)
[1, 2, 3]