Split lines from file

gistfile1.txt

#!/usr/bin/env python

def split_into_groups(iterable, group_size):
    """Split an iterable collection into groups with fixed size.

    Yield
    -----
    list[any]
        Groups of elements.
    """
    buf = []
    it = iter(iterable)
    try:
        while True:
            for i in range(group_size):
                buf.append(next(it))
            yield buf
            del buf[:]
    except StopIteration:
        pass
    if buf:
        yield buf


with open('input.txt', 'rb') as fin:
    for i, lines in enumerate(split_into_groups(fin, group_size=10000)):
        with open('output%d.txt' % i, 'wb') as fout:
            fout.write(b''.join(lines))