python编程中的if __name__ == 'main与windows中使用多进程

if __name__ == 'main

一个python的文件有两种使用的方法，第一是直接做为程序执行，第二是import到其余的python程序中被调用（模块重用）执行。

所以if __name__ == 'main': 的做用就是控制这两种状况执行代码的过程，

在if __name__ == 'main': 下的代码只有在第一种状况下（即文件做为程序直接执行）才会被执行，而import到其余程序中是不会被执行的

第一种状况：

def func():
    print('second')
print('first')

print(__name__)  # 当运行自身被执行的时候，__name__=__main__
if __name__ == '__main__':
    func()


#当单独执行该文件时，即python main_test.py,输出结果：
""" first __main__ second """

if __name__=="__main__": 语句以前和以后的代码都被执行。

第二种状况：

import main_test # 导入main_test文件

if __name__ == '__main__':
    print('main_test2')

# 当单独执行该文件时，即python mian_test2.py,输出结果
""" first # main_test模块中的print main_test # 当main_test被导入时， main_test.py中print(__name__) 输出为导入模块的名称main_test，而不是__main__。 main_test2 """

second没有被输出？

 main_test被导入后，不知足__name__ == '__main__' 因此不会输出second，即main_test中的if __name__=="__main__": 以前的语句被执行，以后的没有被执行

 

Windows建立多进程

代码一

import multiprocessing
import time
import threading

print('main process start1')
def func():
    print('subprocess start',time.asctime(time.localtime(time.time())))
    time.sleep(3)
    print('subprocess end',time.asctime(time.localtime(time.time())))

p = multiprocessing.Process(target=func, name='1号')
p.start()

print('main process end', time.asctime(time.localtime(time.time())),multiprocessing.current_process())
if __name__ == '__main__':
    pass
    # 默认全部进程结束后，程序才结束

执行效果

main process start1 main process end Fri Mar 23 11:04:46 2018 <_MainProcess(MainProcess, started)> main process start1 至关于导入该模块

windows创造进程至关于导入该文件，因为建立进程在main函数以前，会无限递归导入模块建立进程。
因此在windows创造进程应放在main函数以后，由于main函数不会执行被导入模块。

 Traceback (most recent call last): File "<string>", line 1, in <module> File "C:\Program Files\Python36\lib\multiprocessing\spawn.py", line 105, in spawn_main exitcode = _main(fd) File "C:\Program Files\Python36\lib\multiprocessing\spawn.py", line 114, in _main prepare(preparation_data) File "C:\Program Files\Python36\lib\multiprocessing\spawn.py", line 225, in prepare _fixup_main_from_path(data['init_main_from_path']) File "C:\Program Files\Python36\lib\multiprocessing\spawn.py", line 277, in _fixup_main_from_path run_name="__mp_main__") File "C:\Program Files\Python36\lib\runpy.py", line 263, in run_path pkg_name=pkg_name, script_name=fname) File "C:\Program Files\Python36\lib\runpy.py", line 96, in _run_module_code mod_name, mod_spec, pkg_name, script_name) File "C:\Program Files\Python36\lib\runpy.py", line 85, in _run_code exec(code, run_globals) File "C:\Users\china\PycharmProjects\rq3\并发\课堂\windows_process.py", line 22, in <module> p.start() File "C:\Program Files\Python36\lib\multiprocessing\process.py", line 105, in start self._popen = self._Popen(self) File "C:\Program Files\Python36\lib\multiprocessing\context.py", line 223, in _Popen return _default_context.get_context().Process._Popen(process_obj) File "C:\Program Files\Python36\lib\multiprocessing\context.py", line 322, in _Popen return Popen(process_obj) File "C:\Program Files\Python36\lib\multiprocessing\popen_spawn_win32.py", line 33, in __init__ prep_data = spawn.get_preparation_data(process_obj._name) File "C:\Program Files\Python36\lib\multiprocessing\spawn.py", line 143, in get_preparation_data _check_not_importing_main() File "C:\Program Files\Python36\lib\multiprocessing\spawn.py", line 136, in _check_not_importing_main is not going to be frozen to produce an executable.''') RuntimeError: An attempt has been made to start a new process before the current process has finished its bootstrapping phase. This probably means that you are not using fork to start your child processes and you have forgotten to use the proper idiom in the main module:

 代码二

import multiprocessing
import time
import threading

print('main process start1')
def func():
    print('subprocess start',time.asctime(time.localtime(time.time())))
    time.sleep(3)
    print('subprocess end',time.asctime(time.localtime(time.time())))


if __name__ == '__main__': #把进程建立放在main函数中
    p = multiprocessing.Process(target=func, name='1号')
    p.start()
    print('main process end', time.asctime(time.localtime(time.time())), multiprocessing.current_process())
    # 默认全部进程结束后，程序才结束

Windows输出结果

main process start1 main process end Fri Mar 23 11:17:00 2018 <_MainProcess(MainProcess, started)> main process start1 子进程建立时，导入该模块输出main函数以前语句 subprocess start Fri Mar 23 11:17:00 2018 子进程执行func subprocess end Fri Mar 23 11:17:03 2018

 在Linux上输出结果

main process start1 只输出一个start1 main process end Fri Mar 23 12:29:12 2018 <_MainProcess(MainProcess, started)> subprocess start Fri Mar 23 12:29:12 2018 subprocess end Fri Mar 23 12:29:15 2018