# List comprehension for reading lists of integers

As of reading the following input.txt file:

1 6 14
1 2 7
1 3 9
2 3 10
2 4 15
3 6 2
3 4 11
4 5 6
5 6 9


I wanted to make a list of lists containing each line as a list of integers

[[1, 6, 14],
[1, 2, 7],
[1, 3, 9],
...
[5, 6, 9]]


I achieved that by doing the following

with open('input.txt', 'r') as f:
# gets lines as list of strings [['1, 2, 3,', ...]]
nodes_list = [n.split() for n in f.readlines()]
# convert strings inside the lists to int's [[1, 2, 3], ...]
nodes_list = [[int(y[0]), int(y[1]), int(y[2])] for y in nodes_list]


Is there any way that I can achieve the same with a single comprehension list, without the need of having two separated ones? If not, maybe some cleaner way of doing it?

with open('input.txt', 'r') as f:
nodes_list = [[int(item) for item in line.split()] for line in f]


Notice how:

1. rather than iterating over f.readlines(), which creates a list of the lines in the file, we iterate over f, which produces the same result, but as a generator,
2. rather than first creating a potentially huge list of lists of strings, then converting it to a list of lists of ints, only a single row is instantiated at a time.
• I would also add, that [int(item) for ...] is easier to read than list(map(int, ...)) Jan 27, 2016 at 13:45
• Have to apologize to @mleyfman, but your answer is more clear and concise. Thanks. Jan 27, 2016 at 16:42

You can use a map to give you what you want in a single line:

with open('input.txt', 'r') as f:
nodes = [list(map(int, line.split())) for line in f]


An explanation of how it works:

f.readlines() iterates over the lines, with line endings removed

line.split() gives you an iterable over the space-separated characters

map(int, ...) does the conversion to int for every element

list(...) converts it to a list to give you a list of lists

• I have never managed to look at map and filter the same after reading Guido's plans to cut them from Py3K. Jan 27, 2016 at 6:57
• for line in f.readlines() can be rewritten as for line in f (uses a generator) Jan 27, 2016 at 13:39

Just a very small note, that you don't need explicitly use 'r' when opening a file in read mode. That's the default mode, so there's no difference between open('input.txt', 'r') and open('input.txt'). You may as well use the latter for brevity and readability.