Python: understanding/reading list comprehensions

List comprehensions in Python can be tricky and hard to look at but once you know the secret, it will become very clear, for example, say we have a list:

a = [1, 2, 3, 4, 5]

And we want to make a second list out of this “b” with values of “a” plus 2, so we want to get this:

b = [3, 4, 5, 6, 7]

Now, the way to do this without using any fancy feature would be:

b = []
for item in a:
    b.append(item + 2)

To turn this into a list comprehension we will start like this:

b = [for item in a]

Now, we will have to add our “item + 2″ to the left side of the for loop so:

b = [item + 2 for item in a]

If you didn’t understand what happened, don’t worry, just keep reading.
Now, let’s look at another example, this time, we will try to understand a list comprehension that’s already written, a very simple one:

a = [1, 2, 3, 4, 5]
b = [x*x*x for x in a]

To read this, start form right to left:

for x in a: x*x*x

Internally, this will be translated to something like:

b = []
for x in a:
    b.append(x*x*x)

So the secret is to read from right to left.

If there should be a condition, it will come after the for loop so to increment all the items that are greater than 4 by 2:

b = [item + 2 for item in a if item > 4]

I hope this helps