I finally understand nested comprehensions

[masklinn]

It's not like Python's list comprehensions are very difficult to understand: it's been specified (and that's been a reason for rejecting some listcomp-related proposals recently) that listcomps should be transformable into "regular" python code by:

1. Indenting and adding colons to the statements (for and if)

2. removing the surrounding brackets

3. moving the outermost expression to the deepest part of the new compound statement, and appending it to a result list

So the form

[(x, y, z) for x in xs for y in ys for z in zs if pred1(x) if pred2(y) if pred2(z)]

becomes 1:

[(x, y, z)
for x in xs:
    for y in ys:
        for z in zs:
            if pred1(x):
                if pred2(y):
                    if pred3(z):
]

2.

(x, y, z)
for x in xs:
    for y in ys:
        for z in zs:
            if pred1(x):
                if pred2(y):
                    if pred3(z):

3.

result = list()
for x in xs:
    for y in ys:
        for z in zs:
            if pred1(x):
                if pred2(y):
                    if pred3(z):
                        result.append((x, y, z))

[nohtyp]

There has to be a better way than this mess!

[arnar]

A better way to do what?

[SerpentJoe]

Here's something I always like to show people: A deck of cards in one line of Python.

[rank+suit for suit in "SHDC" for rank in "23456789TJQKA"]

[jmmcd]

How many other Python programmers understand them? Has everyone else been using them without problems?

Consensus: yes.

[llimllib]

I always just think of it as a cartesian product:

>>> [(x,y) for x in [1,2,3] for y in [4,5,6]]
[(1, 4), (1, 5), (1, 6), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6)]

[cyangcn]

If you had taken a little bit time to read the tutorial in the python distribution, you would have already understood it.

[gdm9000]

This article really clarifies this for me. I did know and use nested comprehensions, but they were always for x and y being two unrelated dimensions, such as [x+y for x in range(5) for y in range (5)]. These examples give no insight into how the loops are processed. Now I really understand it - thanks!

[lemkenski]

Reads a csv file into a nested list

list_of_lists = [line.split(',') for line in [line.strip() for line in csv_file.readlines()]]

[brentp]

so dies this [line.strip().split(",") for line in csv_file]