How does the list comprehension to flatten a python list work? [duplicate]

Question

Let’s take a look at your list comprehension then, but first let’s start with list comprehension at it’s easiest.

l = [1,2,3,4,5]
print [x for x in l] # prints [1, 2, 3, 4, 5]

You can look at this the same as a for loop structured like so:

for x in l:
    print x

Now let’s look at another one:

l = [1,2,3,4,5]
a = [x for x in l if x % 2 == 0]
print a # prints [2,4]

That is the exact same as this:

a = []
l = [1,2,3,4,5]
for x in l:
    if x % 2 == 0:
        a.append(x)
print a # prints [2,4]

Now let’s take a look at the examples you provided.

l = [[1,2,3],[4,5,6]]
flattened_l = [item for sublist in l for item in sublist]
print flattened_l # prints [1,2,3,4,5,6]

For list comprehension start at the farthest to the left for loop and work your way in. The variable, item, in this case, is what will be added. It will produce this equivalent:

l = [[1,2,3],[4,5,6]]
flattened_l = []
for sublist in l:
    for item in sublist:
        flattened_l.append(item)

Now for the last one

exactly_the_same_as_l = [item for item in sublist for sublist in l]

Using the same knowledge we can create a for loop and see how it would behave:

for item in sublist:
    for sublist in l:
        exactly_the_same_as_l.append(item)

Now the only reason the above one works is because when flattened_l was created, it also created sublist. It is a scoping reason to why that did not throw an error. If you ran that without defining the flattened_l first, you would get a NameError

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