Python: elif or new if?

Question

What is better to use:

if var in X:
    #do_whatever
elif (var in Y):
    #do_whatever2

or:

if var in X:
    #do_whatever
if var in Y:
    #do_whatever2

Assuming var can't be in both X and Y... Is there any rule or common practice? Should I use elif? or a new if? or it doesn't matter??

EDIT: Great answers.. But can I say that if the first statement (#do_whatever) ends with a return or a break; where in its end the other condition will not be tested thus wasting the system resources or causing trouble, it's ok to do whatever.. I guess...

Answer 1

It makes a difference in some cases. See this example:

def foo(var):
    if var == 5:
        var = 6
    elif var == 6:
        var = 8
    else:
        var = 10

    return var

def bar(var):
    if var == 5:
        var = 6
    if var == 6:
        var = 8
    if var not in (5, 6):
        var = 10

    return var

print foo(5)        # 6
print bar(5)        # 10

Answer 2

If the second statement is impossible if the first one is true (as is the case here, since you said var can't be in both X and Y), then you should be using an elif. Otherwise, the second check will still run, which is a waste of system resources if you know it's going to false.