function that takes iterables and returns a string

Question

def alternate(*args):
    l = ''
    c = 0
    while True:
        try:
            iterable = args[c]
            i = iter(iterable)
            l = l + next(i)
            c = c + 1
        except:
            break
    yield l

The alternate generator takes any number of iterables as parameters: it produces the first value from the first parameter, then the first value from the second parameter, ..., then the first value from the last parameter; then the second value from the first parameter, then the second value from the second parameter, ..., then the second value from the last parameter; etc. If any iterable produces no more values, this generator produces no more values. For example:

for i in alternate('abcde','fg','hijk'):
    print(i,end='')

the answer is:

afhbgic

when my function takes

('abcde','fg','hijk')

it returns

afh

but the correct answer is

afhbgic

can someone tell me how to fix it? many thanks!

Answer 1

What about solution like this:

def alternate(*args):
    l = ''

    # initialize iterators for each argument
    iterators = [iter(it) for it in args]

    # iterate over lengths, using iterators
    while True:
        for it in iterators:
            try:
                l = l + next(it)
            except StopIteration:
                return l

    return l


result = alternate('abcde','fg','hijk')
print(result) # afhbgic

Answer 2

If you must have a generator, you can make use of itertools.zip_longest( izip_longest if using Python 2.x):

from itertools import zip_longest


def alternate(*args):
    for tup in zip_longest(*args):
        for el in tup:
            if el:
                yield el
            else:
                return

Output:

>>> ''.join(alternate('abcde','fg','hijk'))
'afhbgic'
>>> 

Otherwise, just use a normal function:

from itertools import zip_longest


def alternate(*args):
    s = ''
    for tup in zip_longest(*args):
        for el in tup:
            if el:
                s += el
            else:
                return s

Output:

>>> alternate('abcde','fg','hijk')
'afhbgic'
>>>