multithreading - Why does this multithreaded python program print from 0 to 99 CORRECTLY? -

this code.

from queue import queue threading import *  threadlock = lock()  def do_stuff(q):   while true:     threadlock.acquire()     print q.get()     q.task_done()     threadlock.release()  q = queue(maxsize=0) num_threads = 10  x in range(100):   q.put(x)  in range(num_threads):   worker = thread(target=do_stuff, args=(q,))   worker.setdaemon(false)   worker.start()    q.join() 

when execute code, numbers 0 99 printed sorted. when remove locks in do_stuff, expect numbers 0 99 printed unsorted, erroneour numbers here , there, prints range sorted again 0 99. why that? shouldn't unsorted, since i'm not synchronizing threads in way?

your function doesn't else between retrieving next number queue , printing it.

python holds lock (the gil) while executing each bytecode, between bytecodes can threads switch. looking @ function (without locks) shows there one spot thread switch give thread chance grab next number , print before preceding thread can print theirs:

>>> import dis >>> def do_stuff(q): ...   while true: ...     print q.get() ...     q.task_done() ...  >>> dis.dis(do_stuff)   2           0 setup_loop              31 (to 34)         >>    3 load_global              0 (true)               6 pop_jump_if_false       33    3           9 load_fast                0 (q)              12 load_attr                1 (get)              15 call_function            0              18 print_item                        19 print_newline           4          20 load_fast                0 (q)              23 load_attr                2 (task_done)              26 call_function            0              29 pop_top                           30 jump_absolute            3         >>   33 pop_block                    >>   34 load_const               0 (none)              37 return_value         

even if thread switched there, thread must complete both call_function and print_item bytecodes before control switched back, see items being printed out of order.

the thread switch have take place between call_function , print_item. if introduced more instructions there, you'd increase chances of numbers being printed out of order.