Remove list from list in Python [duplicate]

Question

Possible Duplicate:
Get difference from two lists in Python

What is a simplified way of doing this? I have been trying on my own, and I can't figure it out. list a and list b, the new list should have items that are only in list a. So:

a = apple, carrot, lemon b = pineapple, apple, tomato new_list = carrot, lemon 

I tried writing code, but every time it always returns the whole list a to me.

Answer 1

You can write this using a list comprehension which tells us quite literally which elements need to end up in new_list:

a = ['apple', 'carrot', 'lemon'] b = ['pineapple', 'apple', 'tomato']  # This gives us: new_list = ['carrot' , 'lemon'] new_list = [fruit for fruit in a if fruit not in b] 

Or, using a for loop:

new_list = [] for fruit in a:     if fruit not in b:         new_list.append(fruit) 

As you can see these approaches are quite similar which is why Python also has list comprehensions to easily construct lists.

Answer 2

You can use a set:

# Assume a, b are Python lists  # Create sets of a,b setA = set(a) setB = set(b)  # Get new set with elements that are only in a but not in b onlyInA = setA.difference(b) 

UPDATE
As iurisilvio and mgilson pointed out, this approach only works if a and b do not contain duplicates, and if the order of the elements does not matter.