import bz2, threading, array

fac = lambda b: array.array("c", b)
fac = bytearray

bz2c = bz2.BZ2Compressor()
# Span at least a whole arena (256KB long)
junk_len = 512 * 1024
junk = fac(b"a") * junk_len
empty = fac(b"")
buf = empty[:]

def f():
    bz2c.compress(buf)

for x in range(50):
    buf[:] = junk
    t = threading.Thread(target=f)
    t.start()
    buf[:] = empty
    t.join()