Nested list comprehension in Python

When you code in python, you will use list comprehension eventually. It's not hard, but when it comes to NESTED list comprehension, it's a bit tricky.

As I go through this over an over, I just write it down to help others.

CODE: (you can copy and paste in your IDE or IDLE)

Note: code syntax highligth does NOT match Python syntax highlight. I use the colors to make the relation between with and without list comprehension

#####################################################
# first list
a = [1, 2, 3, 4]

# second list
b = ['a', 'b']

# expected list
exp = ['a1', 'a2', 'a3', 'a4', 'b1', 'b2', 'b3', 'b4']

# WITHOUT list comprehension
result = []
for item_a in a:
    for item_b in b:
        result.append(item_b+str(item_a))

print('without', result)

# WITH list comprehension
result = [item_b+str(item_a) for item_a in a for item_b in b]

print('with', result)
######################################################

This is the easy one.. but what happens if we have a nested list, let's see..

#####################################################
# first list
a = [ [1, 2], [3, 4] ]

# second list
b = ['a', 'b']

# expected list
exp = ['a1', 'a2', 'a3', 'a4', 'b1', 'b2', 'b3', 'b4']

# WITHOUT list comprehension
result = []
for item_b in b:
    for list_a in a:
        for item_a in list_a:
            result.append(item_b+str(item_a))

print('without', result)

# WITH list comprehension
result = [item_b+str(item_a) for item_b in b for list_a in a for item_a in list_a]

print('with', result)
######################################################

I guess you can see the pattern =)

So, the structure will be:

[ OPERATION for #1 for #2 ... for #N]