未调用join/close的线程问题及多线程资源耗尽风险问询

Hey there! Let's walk through your threading questions clearly—they're really common when you're getting the hang of Python's threading module, so great asking them.

1. What happens if you call thread.start() but never call join() or close()? #

First, let's break it down by thread type:

• For non-daemon threads (the default), the thread will run its target function to completion on its own. Once finished, it enters a "terminated" state—but it doesn't fully clean up system resources right away. These become "zombie threads": they don't use CPU anymore, but hold onto small bits of metadata like thread IDs and exit status. If your main thread runs indefinitely (like a server), accumulating zombies will eventually eat into your system's maximum allowed thread count, blocking you from creating new threads later.
• For daemon threads (set with t1.daemon = True before starting), the main thread won't wait for it to finish. When the main thread exits, the daemon thread gets killed immediately—no zombie state, but you risk cutting off incomplete work the thread was handling.

Either way, skipping join() means you lose the ability to wait for the thread to finish or retrieve its exit status, which can lead to race conditions or unhandled errors in more complex code.

2. If you keep creating threads like this, will you run out of memory or available threads? Is it okay to just call start() and leave them be? #

Short answers: Yes, you will hit resource limits eventually, and it's almost never a good idea to leave threads unmanaged long-term. Here's the breakdown:

• Thread count limits: Every OS caps how many threads a single process can spawn (usually in the thousands, depending on system config and RAM). Nonstop thread creation will hit this limit quickly, throwing a RuntimeError when Python can't spawn more.
• Memory usage: Each thread has its own stack space (several MB by default on most systems). Even for tiny threads like your hello() example, multiplying that stack size by hundreds of threads will eat up RAM fast.
• Resource leaks: If your threads do more than print (e.g., open files, hold database connections, or acquire locks), abandoning them without cleanup can leave those resources open, degrading your program over time.

The only time leaving threads be might be okay is for a tiny script that creates 2-3 short-lived threads and exits immediately. For long-running apps, use proper management: call join() to wait for threads and clean up, or use a thread pool (like concurrent.futures.ThreadPoolExecutor) to reuse threads instead of spawning new ones every time.

内容的提问来源于stack exchange，提问作者Anh Vũ Nguyễn