How to read aloud Python List Comprehensions?

Question

My question is about Python List Comprehension readability. When I come across code with complex/nested list comprehensions, I find that I have to re-read them several times in order to understand the intent.

Is there an intuitive way to read aloud list comprehensions? Seems like I should start "reading" from the middle, then read the if conditions (if any), and read the expression last.

Here's how I would read the follow line of code aloud, in order to understand it:

[(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]

"For each element in List x, and each element in List y, if the two elements are not the same, create a list of tuples."

Two examples that I am struggling with: How would you read the following List Comprehensions aloud?

From another question in Stack Overflow: [x for b in a for x in b]
Python docs has this example: [[row[i] for row in matrix] for i in range(4)]

Any suggestions or pointers for ways to read aloud list comprehensions such that the intention becomes clearer is much appreciated.

don't read programming code aloud, that usually confuses more than it clarifies — Lie Ryan
@LieRyan My question is really around readability and understanding others' code. I wrote "read aloud" because I didn't know how else to phrase it. — Ram Narasimhan
readability doesn't imply reading aloud; I usually try to categorize the comprehension's loop form. There are three general ways to nest comprehensions, the [... for ... in A for ... in B] pattern forms a cartesian product; the [... for ... in [... for ... in A]] forms a pipeline (as in shell pipes); and the [[... for ... in A] for ... in B] creates a multidimensional array. After categorizing the looping's general form, interpreting the expression and if-condition part becomes trivial. — Lie Ryan
Since list comprehension follows the mathematical set-builder notation, in the more simple cases it may be helpful to grasp it using this notation itself: S = {(x, y) : x ∈ {1,2,3} and y ∈ {3,1,4}, x != y}. In more complex examples it's not very helpful, though. — OfirD

AndiDog AndiDog · Accepted Answer · 2012-01-30T09:34:50

I usually unfold it in my mind into a generating loop, so for example

[(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]

is the list comprehension for the generator

for x in [1,2,3]:
    for y in [3,1,4]:
        if x != y:
            yield (x, y)

Example #1

[x for b in a for x in b] is the comprehension for

for b in a:
    for x in b:
        yield x

Example result for a = [[1,2,3],[4,5,6]]: [1, 2, 3, 4, 5, 6]

Example #2

[[row[i] for row in matrix] for i in range(4)] (note the inner expression is another comprehension!):

for i in range(4):
    yield [row[i] for row in matrix]

which is unfolded

for i in range(4):
    l = []

    for row in matrix:
        l.append(row[i])

    yield l

How to read aloud Python List Comprehensions?

2 Answers

Example #1

Example #2