Getting first n unique elements from Python list

Question

I have a python list where elements can repeat.

>>> a = [1,2,2,3,3,4,5,6]

I want to get the first n unique elements from

Answer 1

If your objects are hashable (ints are hashable) you can write utility function using fromkeys method of collections.OrderedDict class (or starting from Python3.7 a plain dict, since they became officially ordered) like

from collections import OrderedDict


def nub(iterable):
    """Returns unique elements preserving order."""
    return OrderedDict.fromkeys(iterable).keys()

and then implementation of iterate can be simplified to

from itertools import islice


def iterate(itr, upper=5):
    return islice(nub(itr), upper)

or if you want always a list as an output

def iterate(itr, upper=5):
    return list(nub(itr))[:upper]

Improvements

As @Chris_Rands mentioned this solution walks through entire collection and we can improve this by writing nub utility in a form of generator like others already did:

def nub(iterable):
    seen = set()
    add_seen = seen.add
    for element in iterable:
        if element in seen:
            continue
        yield element
        add_seen(element)