<s>[INST] Flatten (an irregular) list of lists in Python Yes, I know this subject has been covered before (here, here, here, here), but as far as I know, all solutions, except for one, fail on a list like this:
L = [[[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

flatten(L)

Is this the best model? Did I overlook something? Any problems?
 [/INST] Using generator functions can make your example a little easier to read and probably boost the performance.
Python 2
def flatten(l):
    for el in l:
        if isinstance(el, collections.Iterable) and not isinstance(el, basestring):
            for sub in flatten(el):
                yield sub
        else:
            yield el

I used the Iterable ABC added in 2.6.
Python 3
In Python 3, the basestring is no more, but you can use a tuple of str and bytes to get the same effect there.
The yield from operator returns an item from a generator one at a time. This syntax for delegating to a subgenerator was added in 3.3
def flatten(l):
    for el in l:
        if isinstance(el, collections.Iterable) and not isinstance(el, (str, bytes)):
            yield from flatten(el)
        else:
            yield el

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