python compare difference between two lists

Question

I have two lists like this:

newList = (
    (1546, 'John'),
    (8794, 'Michael'),
    (892416, 'Dave'),
    (456789, 'Lucy'),
    )

oldList = (
    (1546, 'John'),
    (8794, 'Michael'),
    (892416, 'Dave'),
    (246456, 'Alexander')
    )

I would like to have a function comparing both lists. It would be something like this:

def compare(new, old):
    print('Alexander is not anymore in the new list !')
    print('Lucy is new !')
    return newList 

I would like to compare with id of every person which is unique.

Edit: The result would be my function compare. It prints the difference. I don't know how to start

Answer 1

You can convert your lists into sets and take the differences

n = set(l[1] for l in newList)
o = set(l[1] for l in oldList)
print n - o # set(['Lucy'])
print o - n # set(['Alexander'])

Answer 2

edit: I wrote this before I know a lot about sets. Now I would recommend the solution using sets given below.

One solution:

removed = [o for o in old if o[0] not in [n[0] for n in new]]
added = [n for n in new if n[0] not in [o[0] for o in old]]

Or if you present your data as dictionaries:

old = dict(old) # if you do go for this approach be sure to build these
new = dict(new) # variables as dictionaries, not convert them each time

removed = {k:old[k] for k in old.keys() - new.keys()}
added = {k:new[k] for k in new.keys() - old.keys()}

Both turned into functions, returning items in ys but not in xs :

def tuple_list_additions(xs,ys):
  return [y for y in ys if y[0] not in [x[0] for x in xs]]

def dict_additions(xs,ys):
  return {k:ys[k] for k in ys.keys() - xs.keys()}

Answer 3

You can use set :

def compare(old, new):
    oldSet = set(old)
    newSet = set(new)
    removedElements =  oldSet - newSet
    newElements = newSet - oldSet
    for element in removedElements:
        print(element[1] + " is not anymore in the new list!")
    for element in newElements:
        print(element[1] + " is new!")

It's method which compares whole elements (id, name), so if you want to compare only id you should do some modifications (and for example use dicts).