why the objlock doesn't work? (i.e. it should count to 10000)

locktest

import time
import threading

class objnolock(object):
    def __init__(self,c):
        self.c=c
    def inc(self):
        new = self.c+1 
        time.sleep(0.001)
        self.c = new

class objlock(object):
    def __init__(self,c):
        self.c=c
        self.lock = threading.Lock()
    def inc(self):
	with self.lock:
		new = self.c+1 
		time.sleep(0.001)
		self.c = new
	
nolock = objnolock(0)
lock = objlock(0)
for _ in range(10000):
    threading.Thread(target=nolock.inc).start()
    threading.Thread(target=lock.inc).start()
print "with lock", nolock.c  # wrong result. about 20% of the answer.
print "no lock", lock.c      # also wrong results.