zip_longest is problematic, since any solution would silently drop the fillvalue if it occurs in the inputs (this can be worked around, but it's always going to be a little hacky).
The most general solution is the roundrobin recipe from the itertools module:
from itertools import cycle, islice
def roundrobin(*iterables):
"roundrobin('ABC', 'D', 'EF') --> A D E B F C"
# Recipe credited to George Sakkis
num_active = len(iterables)
nexts = cycle(iter(it).__next__ for it in iterables)
while num_active:
try:
for next in nexts:
yield next()
except StopIteration:
# Remove the iterator we just exhausted from the cycle.
num_active -= 1
nexts = cycle(islice(nexts, num_active))
For your input, you'd do something like:
mylist = [
[4,7,9,10],
[5,14,55,24,121,56, 89,456, 678],
[100, 23, 443, 34, 1243,]
....
]
print(list(roundrobin(*mylist)))
