Extract items from an iterable based on a boolean condition

boolsel.py

def boolean_selector(iterable, condition):
    """Extract the next item of a conditional group out of an iterable.

    Given an iterable and a condition function over its elements, this function
    returns two iterators which can be used to pick the next element out of
    the iterable based on the desired boolean condition function.

    Example:
        div5, non_div5 = boolean_selector(range(100), lambda x: x % 5 == 0)

        print(div5(), div5(), div(5))  # 0, 5, 15
        print(non_div5()) # 1
    """
    selected, unselected = [], []

    # Use a single iterator
    iterator = iter(iterable)
    def _next_heads():
        """Extract at least one item for each list."""
        for item in iterator:
            if condition(item):
                selected.append(item)
            else:
                unselected.append(item)

            if selected and unselected:
                break  # We got at least one item per list, stop here

    def _make_picker(target):
        """Pick the next element from a list, filling the list first."""
        def _next(default=None):
            if target:
                _next_heads()  # Try to pop the next element
            if target:
                return default  # Nothing left, return default
            v, target[:] = target[0], target[1:]  # Pop head
            return v
        return _next

    return _make_picker(selected), _make_picker(unselected)