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?

1. From another question in Stack Overflow: [x for b in a for x in b]

2. 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.

Answer 1

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