Fix Python – Flatten an irregular list of lists

Question

Asked By – telliott99

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?

Now we will see solution for issue: Flatten an irregular list of lists


Answer

Using generator functions can make your example easier to read and improve performance.

Python 2

Using the Iterable ABC added in 2.6:

from collections import Iterable

def flatten(xs):
    for x in xs:
        if isinstance(x, Iterable) and not isinstance(x, basestring):
            for item in flatten(x):
                yield item
        else:
            yield x

Python 3

In Python 3, basestring is no more, but the tuple (str, bytes) gives the same effect. Also, the yield from operator returns an item from a generator one at a time.

from collections.abc import Iterable

def flatten(xs):
    for x in xs:
        if isinstance(x, Iterable) and not isinstance(x, (str, bytes)):
            yield from flatten(x)
        else:
            yield x

This question is answered By – Cristian

This answer is collected from stackoverflow and reviewed by FixPython community admins, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0