import os
import threading

NUM_THREADS = 10
NUM_WRITES = 100
WRITE_LEN = 1000

path = 'out.txt'
open(path, 'w').close()

def thr(i):
  b = bytes((ord('a') + i,))
  with open(path, 'ab', buffering=0) as out:
    for _ in range(NUM_WRITES):
      out.write(b * WRITE_LEN)

ts = []
for i in range(NUM_THREADS):
  t = threading.Thread(target=thr, args=(i,))
  t.start()
  ts.append(t)

for t in ts:
  t.join()

with open(path, 'rb') as inp:
  size = os.fstat(inp.fileno()).st_size
  if size != NUM_THREADS * NUM_WRITES * WRITE_LEN:
    print('Wrong size:', size)
  else:
    for _ in range(NUM_THREADS * NUM_WRITES):
      b = inp.read(WRITE_LEN)
      if b != b[:1] * WRITE_LEN:
        print('Mixed data:', b)
        break
    else:
      print('OK')