Question

I got a function like

def f():
    ...
    ...
    return [list1, list2]

this returns a list of lists

[[list1.item1,list1.item2,...],[list2.item1,list2.item2,...]]

now when I do the following:

for i in range(0,2):print f()[i][0:10]

it works and print the lists sliced

but if i do

print f()[0:2][0:10]

then it prints the lists ignoring the [0:10] slicing.

Is there any way to make the second form work or do I have to loop every time to get the desired result?

Answer 1

The reason why these two behave differently is because f()[0:2][0:10] works like this:

1. f() gives you a list of lists.
2. [0:2] gives you a list containing the first two elements in the list of lists. Since the elements in the list of lists are lists, this is also a list of lists.
3. [0:10] gives you a list containing the first ten elements in the list of lists that was produced in step 2.

In other words, f()[0:2][0:10] starts with a list of lists, then takes a sublist of that list of lists (which is also a list of lists), and then takes a sublist of the second list of lists (which is also a list of lists).

In contrast, f()[i] actually extracts the i-th element out of your list of lists, which is just a simple list (not a list of lists). Then, when you apply [0:10], you are applying it to the simple list that you got from f()[i] and not to a list of lists.

The bottom line is that any solution that gives the desired behavior will have to access a single array element like [i] at some point, rather than working only with slices like [i:j].