Question

Here’s the area of code I’m having problems with. T1 and T3 are both lists of lists:

for num in T1:
    print num
    print T3[0]
    print type(num)
    print type(T3[0])
    if num == T3[0]:
        print 'they are the same!'
    else:
        print 'nope they are not!'
    if T3.index(num):
            print 'number exists in list!'

Here’s the result:

[0, 0]
[0, 0]
<type 'list'>
<type 'list'>
they are the same!

The values and types are the same but when I don’t see ‘number exists in list!’ When I tested list_name.index(value) on the command line it works but not here. What am I doing wrong?

Answer 1

The method list.index() returns the index of its argument in the list, and raises a ValueError if the argument isn’t found. The check

if T3.index(num):

basically tests if the index of the argument is non-zero, but it happens to be zero in your example, so the check fails. You probably want to test if the item is contained in the list — use

if num in T3:

to that end.