Python线程join和setDaemon

看一下线程的setDaemon()方法

import time
import threading
import ctypes
import inspect

def sayHello():
    for i in range(10):
        print("hello")
        time.sleep(1)

def _async_raise(tid, exctype):
    """raises the exception, performs cleanup if needed"""
    tid = ctypes.c_long(tid)
    if not inspect.isclass(exctype):
        exctype = type(exctype)
    res = ctypes.pythonapi.PyThreadState_SetAsyncExc(tid, ctypes.py_object(exctype))
    if res == 0:
        raise ValueError("invalid thread id")
    elif res != 1:
        # """if it returns a number greater than one, you're in trouble,
        # and you should call it again with exc=NULL to revert the effect"""
        ctypes.pythonapi.PyThreadState_SetAsyncExc(tid, None)
        raise SystemError("PyThreadState_SetAsyncExc failed")

def stop_thread(thread):
    _async_raise(thread.ident, SystemExit)

if __name__ == '__main__':
    # sayHello()
    t = threading.Thread(target=sayHello, args=())
    t.setDaemon(True) # 主线程结束后停止子线程
    t.start()
    for i in range(3):
        print(t.is_alive())
        time.sleep(1)

上面的输出是：

hello
True
True
hello
hello
True
hello

我们修改一下代码：

import time
import threading
import ctypes
import inspect

def sayHello():
    for i in range(10):
        print("hello")
        time.sleep(1)

def _async_raise(tid, exctype):
    """raises the exception, performs cleanup if needed"""
    tid = ctypes.c_long(tid)
    if not inspect.isclass(exctype):
        exctype = type(exctype)
    res = ctypes.pythonapi.PyThreadState_SetAsyncExc(tid, ctypes.py_object(exctype))
    if res == 0:
        raise ValueError("invalid thread id")
    elif res != 1:
        # """if it returns a number greater than one, you're in trouble,
        # and you should call it again with exc=NULL to revert the effect"""
        ctypes.pythonapi.PyThreadState_SetAsyncExc(tid, None)
        raise SystemError("PyThreadState_SetAsyncExc failed")

def stop_thread(thread):
    _async_raise(thread.ident, SystemExit)

if __name__ == '__main__':
    # sayHello()
    t = threading.Thread(target=sayHello, args=())
    t.setDaemon(False) # 主线程结束后不停止子线程
    t.start()
    for i in range(3):
        print(t.is_alive())
        time.sleep(1)

程序的输出是：

hello
True
True
hello
hello
True
hello
hello
hello
hello
hello
hello
hello

可见，setDaemon()方法就是决定在主线程结束后是否结束子线程，如果为True时，会结束子线程，为False时，不会结束子线程。

我们再来看join()方法：

直接看代码

import time
import threading
import ctypes
import inspect

def sayHello():
    for i in range(10):
        print("hello")
        time.sleep(1)

def _async_raise(tid, exctype):
    """raises the exception, performs cleanup if needed"""
    tid = ctypes.c_long(tid)
    if not inspect.isclass(exctype):
        exctype = type(exctype)
    res = ctypes.pythonapi.PyThreadState_SetAsyncExc(tid, ctypes.py_object(exctype))
    if res == 0:
        raise ValueError("invalid thread id")
    elif res != 1:
        # """if it returns a number greater than one, you're in trouble,
        # and you should call it again with exc=NULL to revert the effect"""
        ctypes.pythonapi.PyThreadState_SetAsyncExc(tid, None)
        raise SystemError("PyThreadState_SetAsyncExc failed")

def stop_thread(thread):
    _async_raise(thread.ident, SystemExit)

if __name__ == '__main__':
    # sayHello()
    t = threading.Thread(target=sayHello, args=())
    t.setDaemon(False) # 主线程结束后不停止子线程
    t.start()
    for i in range(3):
        print(t.is_alive())
        time.sleep(1)
    # t.join()
    print("over")

输出结果为：

hello
True
True
hello
True
hello
hello
over
hello
hello
hello
hello
hello
hello

可以看到主线程结束时，打印出over，之后子线程还在继续打印hello

修改代码：

import time
import threading
import ctypes
import inspect

def sayHello():
    for i in range(10):
        print("hello")
        time.sleep(1)

def _async_raise(tid, exctype):
    """raises the exception, performs cleanup if needed"""
    tid = ctypes.c_long(tid)
    if not inspect.isclass(exctype):
        exctype = type(exctype)
    res = ctypes.pythonapi.PyThreadState_SetAsyncExc(tid, ctypes.py_object(exctype))
    if res == 0:
        raise ValueError("invalid thread id")
    elif res != 1:
        # """if it returns a number greater than one, you're in trouble,
        # and you should call it again with exc=NULL to revert the effect"""
        ctypes.pythonapi.PyThreadState_SetAsyncExc(tid, None)
        raise SystemError("PyThreadState_SetAsyncExc failed")

def stop_thread(thread):
    _async_raise(thread.ident, SystemExit)

if __name__ == '__main__':
    # sayHello()
    t = threading.Thread(target=sayHello, args=())
    t.setDaemon(False) # 主线程结束后不停止子线程
    t.start()
    for i in range(3):
        print(t.is_alive())
        time.sleep(1)
    t.join()
    print("over")

输出结果为：

hello
True
hello
True
True
hello
hello
hello
hello
hello
hello
hello
hello
over

可以看到设置t.join()方法之后，主线程要等待t这个线程结束之后，才能继续，也就是等hello打印完之后才打印over。