def collapse(iterable, base_type=None, levels=None):
    """Flatten an iterable with multiple levels of nesting (e.g., a list of
    lists of tuples) into non-iterable types.

        >>> iterable = [(1, 2), ([3, 4], [[5], [6]])]
        >>> list(collapse(iterable))
        [1, 2, 3, 4, 5, 6]

    Binary and text strings are not considered iterable and
    will not be collapsed.

    To avoid collapsing other types, specify *base_type*:

        >>> iterable = ['ab', ('cd', 'ef'), ['gh', 'ij']]
        >>> list(collapse(iterable, base_type=tuple))
        ['ab', ('cd', 'ef'), 'gh', 'ij']

    Specify *levels* to stop flattening after a certain level:

    >>> iterable = [('a', ['b']), ('c', ['d'])]
    >>> list(collapse(iterable))  # Fully flattened
    ['a', 'b', 'c', 'd']
    >>> list(collapse(iterable, levels=1))  # Only one level flattened
    ['a', ['b'], 'c', ['d']]

    """
    def walk(node, level):
        if (
            ((levels is not None) and (level > levels)) or
            isinstance(node, (str, bytes)) or
            ((base_type is not None) and isinstance(node, base_type))
        ):
            yield node
            return

        try:
            tree = iter(node)
        except TypeError:
            yield node
            return
        else:
            for child in tree:
                for x in walk(child, level + 1):
                    yield x

    yield from walk(iterable, 0)
Flatten an iterable with multiple levels of nesting (e.g., a list of
    lists of tuples) into non-iterable types.

        >>> iterable = [(1, 2), ([3, 4], [[5], [6]])]
        >>> list(collapse(iterable))
        [1, 2, 3, 4, 5, 6]

    Binary and text strings are not considered iterable and
    will not be collapsed.

    To avoid collapsing other types, specify *base_type*:

        >>> iterable = ['ab', ('cd', 'ef'), ['gh', 'ij']]
        >>> list(collapse(iterable, base_type=tuple))
        ['ab', ('cd', 'ef'), 'gh', 'ij']

    Specify *levels* to stop flattening after a certain level:

    >>> iterable = [('a', ['b']), ('c', ['d'])]
    >>> list(collapse(iterable))  # Fully flattened
    ['a', 'b', 'c', 'd']
    >>> list(collapse(iterable, levels=1))  # Only one level flattened
    ['a', ['b'], 'c', ['d']]