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?
Using generator functions can make your example a little easier to read and probably boost the performance.
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.
In Python 3, the
basestring is no more, but you can use a tuple of
bytes to get the same effect there.
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