Effective Python Memo Item 8 Avoid three or more expressions in list comprehensions

** Notes on Effective Python ** Item 8: Avoid more than one expression in list comprehensions (p16-18)

Two or more for statements can be written in the comprehension notation

#Output the contents of the matrix to one list
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flat = [x for row in matrix for x in row]
print(flat)

>>>
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Convenient! Let's do a little complicated processing

#Leave the procession and square all the contents
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
squared = [[x ** 2 for x in row] for row in matrix]
print(squared)

>>>
[[1, 4, 9], [16, 25, 36], [49, 64, 81]]

Yeah, it's a little complicated but still readable Let's add another loop!

#Output the contents of a 3D matrix into a single list
my_list = [[[1, 2, 3], [4, 5, 6]], [[7, 8, 9], [10, 11, 12]]]
flat = [x for sublist1 in my_list for sublist2 in sublist1 for x in sublist2]
print(flat)

>>>
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

It became very hard to see. At this point, there is almost no merit of inclusion notation. Let's write with a normal for statement

#Write in a normal for loop
my_list = [[[1, 2, 3], [4, 5, 6]], [[7, 8, 9], [10, 11, 12]]]
flat = []
for sublist1 in my_list:
    for sublist2 in sublist1:
        flat.extend(sublist2)   #It's not append!

print(flat)

>>>
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

This will have longer lines, but it will be more readable than the previous description. The most important thing is readability rather than the number of lines!