How can I flatten lists without splitting strings?

Solution:

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.)

A slight modification of orip's answer that avoids creating an intermediate list:

import itertools
items = ['cat',['dog','bird']]
itertools.chain.from_iterable(itertools.repeat(x,1) if isinstance(x,str) else x for x in items)