getting the index while interating a list

Question

while i'm interating a list, how can i get the id of the current item to reference it to list methods?

xl = [1,2,3] # initial list
yl = [3,2]   # list used to remove items from initial list

for x in xl[:]:
    for y in yl:
        if x == y:
            xl.pop(x) # problem
            break
        print x, y
print xl

in the simple example, i want to loop through the 2 lists, and when i find a similar item, remove it from list 1.

What should i use instead of X in the line commented with "#problem"?

PS: Note it's a copy i'm iterating from.

Answer 1

The general way to do this is with enumerate.

for idx, item in enumerate(iterable):
  pass

But for your use case, this is not very pythonic way to do what you seem to be trying. Iterating over a list and modifying it at the same time should be avoided. Just use a list comprehension:

xl = [item for item in xl if item not in yl]

Answer 2

How about xl = [x for x in xl if x not in y]

This is known as a list comprehension.

Answer 3

Instead of removing elements you don't like, you should simply use filter:

x1 = filter(x1, lambda x: x not in y1)

Alternatively, a list comprehension works as well:

x1 = [x for x in x1 if x not in y1]

If y1 is very large, you should look up in a set, like this:

y1set = set(y1)
x1 = filter(x1, lambda x: x not in y1set)

For reference, pop takes an index, and the generic way to get an index is enumerate. However, in nearly all cases, there's a shorter and cleaner way to write the code than using indices.