Python flatten

flatten.py

def flatten(x, depth=-1):
    """Flattens a list of lists into a single list."""
    if depth == 0:
        return x
    if isinstance(x, list):
        result = []
        for el in x:
            if hasattr(el, '__iter__') and not isinstance(el, str):
                result.extend(flatten(el, depth - 1))
            else:
                result.append(el)
        return result
    elif isinstance(x, tuple):
        result = ()
        for el in x:
            if hasattr(el, '__iter__') and not isinstance(el, str):
                sub = flatten(el, depth - 1) if depth < 0 or depth > 0 else el
                result += sub
            else:
                result += (el,)
        return result
    return x