Does Python have an ordered set?

The answer is no, but you can use collections.OrderedDict from the Python standard library with just keys (and values as None) for the same purpose.

Update: As of Python 3.7 (and CPython 3.6), standard dict is guaranteed to preserve order and is more performant than OrderedDict. (For backward compatibility and especially readability, however, you may wish to continue using OrderedDict.)

Here's an example of how to use dict as an ordered set to filter out duplicate items while preserving order, thereby emulating an ordered set. Use the dict class method fromkeys() to create a dict, then simply ask for the keys() back.

>>> keywords = ['foo', 'bar', 'bar', 'foo', 'baz', 'foo']

>>> list(dict.fromkeys(keywords))
['foo', 'bar', 'baz']

As other answers mention, as for python 3.7+, the dict is ordered by definition. Instead of subclassing OrderedDict we can subclass abc.collections.MutableSet or typing.MutableSet using the dict's keys to store our values.

import itertools
import typing

T = typing.TypeVar("T")

class OrderedSet(typing.MutableSet[T]):
    """A set that preserves insertion order by internally using a dict."""

    def __init__(self, iterable: typing.Iterator[T]):
        self._d = dict.fromkeys(iterable)

    def add(self, x: T) -> None:
        self._d[x] = None

    def discard(self, x: T) -> None:
        self._d.pop(x, None)

    def __contains__(self, x: object) -> bool:
        return self._d.__contains__(x)

    def __len__(self) -> int:
        return self._d.__len__()

    def __iter__(self) -> typing.Iterator[T]:
        return self._d.__iter__()

    def __str__(self):
        return f"{{{', '.join(str(i) for i in self)}}}"

    def __repr__(self):
        return f"<OrderedSet {self}>"

Then just:

x = OrderedSet([1, 2, -1, "bar"])
x.add(0)
assert list(x) == [1, 2, -1, "bar", 0]

I added this code, with some tests, in a small library, so anyone can just pip install it.

As others have said, OrderedDict is a superset of an ordered set in terms of functionality, but if you need a set for interacting with an API and don't need it to be mutable, OrderedDict.keys() is actually an implementation abc.collections.Set:

import random
from collections import OrderedDict, abc

a = list(range(0, 100))
random.shuffle(a)

# True
a == list(OrderedDict((i, 0) for i in a).keys())

# True
isinstance(OrderedDict().keys(), abc.Set)   

The caveats are immutability and having to build up the set like a dict, but it's simple and only uses built-ins.

There is a pip library that does this:

pip install ordered-set

Then you can use it:

from ordered_set import OrderedSet

So i also had a small list where i clearly had the possibility of introducing non-unique values.

I searched for the existence of a unique list of some sort, but then realized that testing the existence of the element before adding it works just fine.

if(not new_element in my_list):
    my_list.append(new_element)

I don't know if there are caveats to this simple approach, but it solves my problem.