python - How can I flatten lists without splitting strings?

Question

I'd like to flatten lists that may contain other lists without breaking strings apart. For example:

In [39]: list( itertools.chain(*["cat", ["dog","bird"]]) )
Out[39]: ['c', 'a', 't', 'dog', 'bird']

and I would like

['cat', 'dog', 'bird']

Answer 1

def flatten(foo):
    for x in foo:
        if hasattr(x, '__iter__') and not isinstance(x, str):
            for y in flatten(x):
                yield y
        else:
            yield x

---

Old version for Python 2.x:

def flatten(foo):
    for x in foo:
        if hasattr(x, '__iter__'):
            for y in flatten(x):
                yield y
        else:
            yield x

(In Python 2.x, strings conveniently didn't actually have an __iter__ attribute, unlike pretty much every other iterable object in Python. Note however that they do in Python 3, so the above code will only work in Python 2.x.)