How do I use a list of numbers as indexes in a loop?

I have a file with multiple single-lines of letter (in this case, DNA sequences).

list = ["AATCGATCGG","GACTCGGTTA","AGGGGTACTA"]

I have indexes of interest that I would like to look at for each line, which I have put in a list.

idx = [7,8,9]

I would like to code to go through each line of list and look at each of the positions in idx within the line. How can I get a list of numbers to be read as individual index values within a loop?
Edit: I would like the output to look similar to what is below:

CGG
TTA
CTA

When I look at a specific index, using the code below, I get output for that position.

for line in list:
print(line[7])

However, when I try to use the list of indexes in place of the specific index, using a variation of the code below:

for line in list:
print(line[idx])

I get errors indicating that the the list items are not being read individually.

"TypeError: byte indices must be integers or slices, not list"
"TypeError: string indices must be integers"

How do I get a list to be iterated through within a for loop?

I am not sure how you want the output formatting. The code below shows two alternatives. Note list has a specific use in Python so is best not used as a name.

my_list = ["AATCGATCGG","GACTCGGTTA","AGGGGTACTA"]

idx = [7,8,9]

for line in my_list:
    my_extract = 
 for i in idx]
    my_string = "".join(my_extract)
    print(my_extract)
    print(my_string)

gives:

['C', 'G', 'G']
CGG
['T', 'T', 'A']
TTA
['C', 'T', 'A']
CTA

You can use list comprehension with a regular for loop.

dna_lists = ["AATCGATCGG","GACTCGGTTA","AGGGGTACTA"]
for line in dna_lists:
    print("".join(
 for i in idx])

The following list comprehension forms a new list using the indexed letters in the sequence.

 for i in idx]

The "".join(list) form is used for creating a string from the list members.

To make sure the items in the list are being read individually you need to do :

for i in list :

or

list = [ ]
b = list[1:2]
c = list[2:3]
d = list[3:4]
e = list[4:5]
f = list[5:6]
g = list[6:7]

print(b,c,d,e,f,g)