Why am I getting "list assignment index out of range" error?

Question

I keep getting the same error:

Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
IndexError: list assignment index out of range

What's wrong with the following code?

myList = [([None] * 8) for x in range(16)]

for i in range(0,7):
    for j in range(0,15):
        myList[i][j] = 2 * i

Answer 1

The list comprehension

myList = [([None] * 8) for x in range(16)]

can be understood like this

mylist = []
for x in range(16):
    mylist.append([None] * 8)

So, you are creating a list of 16 lists, each contains 8 Nones. But with the loops

for i in range(0,7):
    for j in range(0,15):

you are trying to access 15 elements from the first 7 lists. That is why it is failing. Instead, you might want to do

for i in range(16):
    for j in range(8):
        ...

Actually you can do the same in list comprehension, like this

[[2 * i for j in range(8)] for i in range(16)]

Demo:

>>> from pprint import pprint
>>> pprint([[2 * i for j in range(8)] for i in range(16)])
[[0, 0, 0, 0, 0, 0, 0, 0],
 [2, 2, 2, 2, 2, 2, 2, 2],
 [4, 4, 4, 4, 4, 4, 4, 4],
 [6, 6, 6, 6, 6, 6, 6, 6],
 [8, 8, 8, 8, 8, 8, 8, 8],
 [10, 10, 10, 10, 10, 10, 10, 10],
 [12, 12, 12, 12, 12, 12, 12, 12],
 [14, 14, 14, 14, 14, 14, 14, 14],
 [16, 16, 16, 16, 16, 16, 16, 16],
 [18, 18, 18, 18, 18, 18, 18, 18],
 [20, 20, 20, 20, 20, 20, 20, 20],
 [22, 22, 22, 22, 22, 22, 22, 22],
 [24, 24, 24, 24, 24, 24, 24, 24],
 [26, 26, 26, 26, 26, 26, 26, 26],
 [28, 28, 28, 28, 28, 28, 28, 28],
 [30, 30, 30, 30, 30, 30, 30, 30]]

Note: I have used 16 and 8 in the range function because it, by default, starts from 0 and iterates till the parameter passed to it - 1. So, 0 to 15 and 0 to 8 respectively.