How to modify specific item in list of list using list comprehension in python

Question

I am just wondering if there is any better way of modifying the specific item in the list of list using List comprehension? In below example, I am squaring the second item of each list inside list, but, I don't want to eliminate inner list comprehension.

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

nl = [[num**2 if i==1 else num for i, num in enumerate(x)] for x in l]
print nl

Answer 1

Not sure how to keep the inner comprehension but you could do something like this:

def square_idx_one(sl):
    sl[1] **= 2
    return sl

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

nl = [square_idx_one(sl) for sl in l]
print (nl)

result:

[[1, 4, 3], [4, 25, 6], [7, 64, 9]]

But if you're modifying the original I think a for loop probably edges this solution for performance, not to mention memory

Answer 2

In your case, simply

print [[x, y**2, z] for x, y, z in l]

would do the job and says more explicitly what is going on. In general, do

from itertools import izip
p = (1, 2, 1) # element 0 power 1
#             # element 1 power 2
#             # element 2 power 1
# ...
print [[x**power for x, power in izip(row, p)] for row in l]