[QUESTION]code stops on condition_variable.wait

I am trying to implement a thread-safe queue in c++ with std::queue, std::mutex and std::contion_variable. I want to use pybind11 to call it from python. The whole logic is a multiple producers and multiple consumers.
I started consumers first, but found the whole progress was blocked at line 4 in C++(from line 12 in PYTHON), so the producers would not start and it will always be blocked here. I also tried ThreadSafeQueue in c++, the logic is same as python and the code acted normally: producers started, consumers were waken.

is this blocking in python normal? and why? Thanks for reply.

**C++:**
1 int ThreadSafeQueue::wait_pop() {
2    std::unique_lock<std::mutex> lock(_mutex);
3    while (_queue.empty()) {
4        _condition_variable.wait(lock);
5    }
6    int value = _queue.front();
7    _queue.pop();
8    ++_consumer_count;
9    return value;
10 }

**PYTHON:**
1 q = thread_safe_queue.ThreadSafeQueue()
2
3 # multi consumers
4 def consume_func():
5     for i in range(CONSUME_NUM_PER_THREAD):
6         poped_val = q.wait_pop()
7
8 consumer_threads = []
9 for i in range(CONSUMER_NUM):
10    t = threading.Thread(target=consume_func)
11    consumer_threads.append(t)
12    t.start()
13
14 ... ... # producers logic would not start

Start with ThreadSafeQueue::wait_pop and the Python consume_func shown in the issue, then reproduce the case with consumers started before producers. Compare the condition_variable wait path with the producer startup and pybind11 threading behavior; done means explaining whether the blocking is expected and identifying the cause.