Get difference between two lists

Question

I have two lists in Python, like these:

temp1 = ['One', 'Two', 'Three', 'Four']
temp2 = ['One', 'Two']

I need to create a third list with items from the first list which aren't present in the second one. From the example I have to get:

temp3 = ['Three', 'Four']

Are there any fast ways without cycles and checking?

Answer 1

We can calculate intersection minus union of lists:

temp1 = ['One', 'Two', 'Three', 'Four']
temp2 = ['One', 'Two', 'Five']

set(temp1+temp2)-(set(temp1)&set(temp2))

Out: set(['Four', 'Five', 'Three']) 

Answer 2

This is another solution:

def diff(a, b):
    xa = [i for i in set(a) if i not in b]
    xb = [i for i in set(b) if i not in a]
    return xa + xb

Answer 3

if you want something more like a changeset... could use Counter

from collections import Counter

def diff(a, b):
  """ more verbose than needs to be, for clarity """
  ca, cb = Counter(a), Counter(b)
  to_add = cb - ca
  to_remove = ca - cb
  changes = Counter(to_add)
  changes.subtract(to_remove)
  return changes

lista = ['one', 'three', 'four', 'four', 'one']
listb = ['one', 'two', 'three']

In [127]: diff(lista, listb)
Out[127]: Counter({'two': 1, 'one': -1, 'four': -2})
# in order to go from lista to list b, you need to add a "two", remove a "one", and remove two "four"s

In [128]: diff(listb, lista)
Out[128]: Counter({'four': 2, 'one': 1, 'two': -1})
# in order to go from listb to lista, you must add two "four"s, add a "one", and remove a "two"

Answer 4

Here is an simple way to distinguish two lists (whatever the contents are), you can get the result as shown below :

>>> from sets import Set
>>>
>>> l1 = ['xvda', False, 'xvdbb', 12, 'xvdbc']
>>> l2 = ['xvda', 'xvdbb', 'xvdbc', 'xvdbd', None]
>>>
>>> Set(l1).symmetric_difference(Set(l2))
Set([False, 'xvdbd', None, 12])

Hope this will helpful.

Answer 5

If you run into TypeError: unhashable type: 'list' you need to turn lists or sets into tuples, e.g.

set(map(tuple, list_of_lists1)).symmetric_difference(set(map(tuple, list_of_lists2)))

See also How to compare a list of lists/sets in python?

Answer 6

The difference between two lists (say list1 and list2) can be found using the following simple function.

def diff(list1, list2):
    c = set(list1).union(set(list2))  # or c = set(list1) | set(list2)
    d = set(list1).intersection(set(list2))  # or d = set(list1) & set(list2)
    return list(c - d)

or

def diff(list1, list2):
    return list(set(list1).symmetric_difference(set(list2)))  # or return list(set(list1) ^ set(list2))

By Using the above function, the difference can be found using diff(temp2, temp1) or diff(temp1, temp2). Both will give the result ['Four', 'Three']. You don't have to worry about the order of the list or which list is to be given first.

Python doc reference