hmm.py

from threading import Thread


fname = 'testslice'
expected = '\x00\x00\x00\x00a\x00\x00\x00b\x00\x00\x00c\x00\x00\x00d'
slice = 4

open(fname, 'w').close()


def get_slice(idx):
    offset = idx * slice
    f = open(fname, 'r+')
    f.seek(0)
    f.seek(offset + slice - 1)
    f.write('\x00')
    f.seek(0)
    f.seek(offset + slice)
    return f


def write_byte(idx, b):
    with get_slice(idx) as f:
        f.write(b)


a = Thread(target=lambda: write_byte(0, 'a'))
b = Thread(target=lambda: write_byte(1, 'b'))
c = Thread(target=lambda: write_byte(2, 'c'))
d = Thread(target=lambda: write_byte(3, 'd'))
a.start()
b.start()
c.start()
d.start()
a.join()
b.join()
c.join()
d.join()


print repr(open(fname).read())
assert open(fname).read() == expected