What is the difference between chain and chain.from_iterable in itertools?

The first takes 0 or more arguments, each an iterable, the second one takes one argument which is expected to produce the iterables:

from itertools import chain

chain(list1, list2, list3)

iterables = [list1, list2, list3]
chain.from_iterable(iterables)

but iterables can be any iterator that yields the iterables:

def gen_iterables():
    for i in range(10):
        yield range(i)

itertools.chain.from_iterable(gen_iterables())

Using the second form is usually a case of convenience, but because it loops over the input iterables lazily, it is also the only way you can chain an infinite number of finite iterators:

def gen_iterables():
    while True:
        for i in range(5, 10):
            yield range(i)

chain.from_iterable(gen_iterables())

The above example will give you a iterable that yields a cyclic pattern of numbers that will never stop, but will never consume more memory than what a single range() call requires.