Flatten a sequence in Python 3

flatten.py

#!/usr/bin/env python3


def flatten(myseq):
    """Lazily flatten a given sequence."""
    for x in myseq:
        if isinstance(x, (list, tuple, set, dict)):
            yield from flatten(x)
        else:
            yield x


if __name__ == '__main__':
    nested = [4, 5, 'hello', [6, 7, [8, 4, 3, 6],
              2, [5, 7, [3, 2, 6], 2], 12], 42, [3, 5]]
    # Explicitly want a flattened list.
    flattened = list(flatten(nested))

    # Sanity checks.
    assert nested != flattened
    assert len(flattened) > len(nested)

    print('nested:', nested)
    print('flattened:', flattened)