It is working fine, its just that its printing too fast for you to see . Try putting a time.sleep() in both functions (a small amount) to sleep the thread for that much amount of time, to actually be able to see both 1 as well as 2.

Example -

import threadingimport timesem = threading.Semaphore()def fun1():    while True:        sem.acquire()        print(1)        sem.release()        time.sleep(0.25)def fun2():    while True:        sem.acquire()        print(2)        sem.release()        time.sleep(0.25)t = threading.Thread(target = fun1)t.start()t2 = threading.Thread(target = fun2)t2.start()

Also, you can use Lock/mutex method as follows:

import threadingimport timemutex = threading.Lock()  # is equal to threading.Semaphore(1)def fun1():    while True:        mutex.acquire()        print(1)        mutex.release()        time.sleep(.5)def fun2():    while True:        mutex.acquire()        print(2)        mutex.release()        time.sleep(.5)t1 = threading.Thread(target=fun1).start()t2 = threading.Thread(target=fun2).start()

Simpler style using "with":

import threadingimport timemutex = threading.Lock()  # is equal to threading.Semaphore(1)def fun1():    while True:        with mutex:            print(1)        time.sleep(.5)def fun2():    while True:        with mutex:            print(2)        time.sleep(.5)t1 = threading.Thread(target=fun1).start()t2 = threading.Thread(target=fun2).start()

[NOTE]:

The difference between mutex, semaphore, and lock

In fact, I want to find asyncio.Semaphores, not threading.Semaphore,and I believe someone may want it too.

So, I decided to share the asyncio.Semaphores, hope you don't mind.

from asyncio import (    Task,    Semaphore,)import asynciofrom typing import Listasync def shopping(sem: Semaphore):    while True:        async with sem:            print(shopping.__name__)        await asyncio.sleep(0.25)  # Transfer control to the loop, and it will assign another job (is idle) to run.async def coding(sem: Semaphore):    while True:        async with sem:            print(coding.__name__)        await asyncio.sleep(0.25)async def main():    sem = Semaphore(value=1)    list_task: List[Task] = [asyncio.create_task(_coroutine(sem)) for _coroutine in (shopping, coding)]    """     # Normally, we will wait until all the task has done, but that is impossible in your case.    for task in list_task:        await task    """    await asyncio.sleep(2)  # So, I let the main loop wait for 2 seconds, then close the program.asyncio.run(main())

output

shoppingcodingshoppingcodingshoppingcodingshoppingcodingshoppingcodingshoppingcodingshoppingcodingshoppingcoding

16*0.25 = 2