Flatten an irregular (arbitrarily nested) list of lists

Yes, I know this subject has been covered before:

• Python idiom to chain (flatten) an infinite iterable of finite iterables?
• Flattening a shallow list in Python
• Comprehension for flattening a sequence of sequences?
• How do I make a flat list out of a list of lists?

but as far as I know, all solutions, except for one, fail on a list like [[[1, 2, 3], [4, 5]], 6], where the desired output is [1, 2, 3, 4, 5, 6] (or perhaps even better, an iterator).

The only solution I saw that works for an arbitrary nesting is found in this question:

def flatten(x):
    result = []
    for el in x:
        if hasattr(el, "__iter__") and not isinstance(el, basestring):
            result.extend(flatten(el))
        else:
            result.append(el)
    return result

Is this the best approach? Did I overlook something? Any problems?