I frequently find myself doing the following for lists, sets, and dicts.

````
passes = [x for x in seq if cond(x)]
fails = [x for x in seq if not cond(x)]
````

The proposed function would behave similarly to `filter`, but it would return a tuple of the passes iterable and fails iterable rather than just the passes.

````
my_list = [-3, -2, -1, 0, 1, 2, 3]
def is_positive(n):
    return n > 0

positives, negatives = separate(function=is_positive, iterable=my_list)
````

FWIW, here's a recipe from the itertools docs:

    def partition(pred, iterable):
        "Use a predicate to partition entries into false entries and true entries"
        # partition(is_odd, range(10)) --> 0 2 4 6 8   and  1 3 5 7 9
        t1, t2 = tee(iterable)
        return filterfalse(pred, t1), filter(pred, t2)


Also, here's a more general solution that can handle multiple categories:

    >>> from collections import defaultdict
    >>> def categorize(func, iterable):
            d = defaultdict(list)
            for x in iterable:
                d[func(x)].append(x)
            return dict(d)

    >>> categorize(is_positive, [-3, -2, -1, 0, 1, 2, 3])
    {False: [-3, -2, -1, 0], True: [1, 2, 3]}
    >>> categorize(lambda x: x%3, [-3, -2, -1, 0, 1, 2, 3])
    {0: [-3, 0, 3], 1: [-2, 1], 2: [-1, 2]}


At one point, Peter Norvig suggested adding a groupby classmethod to dictionaries.  I would support that idea.   Something like:

    dict.groupby(attrgetter('country'), conferences)

The drawback of that recipe of partition() is that it calls predicate twice on every element. The following implementation calls it only once for every element:

    def partition(pred, iterable):
        t1, t2 = tee((bool(pred(x)), x) for x in iterable)
        return (x for p, x in t1 if not p), (x for p, x in t1 if p)

but it may be less efficient for fast predicates.

Note that the `more_itertools` package on pypi also has a partition method.

Shall we close this off given that a recipe is already present and this is available as part of a popular 3rd-party library?