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，第一是直接作为程序执行，第二是import到其他的python程序中被调用（模块重用）执行。

因此if __name__ == 'main': 的作用就是控制这两种情况执行代码的过程，

在if __name__ == 'main': 下的代码只有在第一种情况下（即文件作为程序直接执行）才会被执行，而import到其他程序中是不会被执行的

第一种情况：

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__": 语句之前和之后的代码都被执行。

第二种情况：

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，不满足__name__ == '__main__' 所以不会输出second，即main_test中的if __name__=="__main__": 之前的语句被执行，之后的没有被执行

Windows创建多进程

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
    # 默认所有进程结束后，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 相当于导入该模块

windows创造进程相当于导入该文件，由于创建进程在main函数之前，会无限递归导入模块创建进程。
所以在windows创造进程应放在main函数之后，因为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 FilesPython36libmultiprocessingspawn.py", line 105, in spawn_main
    exitcode = _main(fd)
  File "C:Program FilesPython36libmultiprocessingspawn.py", line 114, in _main
    prepare(preparation_data)
  File "C:Program FilesPython36libmultiprocessingspawn.py", line 225, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "C:Program FilesPython36libmultiprocessingspawn.py", line 277, in _fixup_main_from_path
    run_name="__mp_main__")
  File "C:Program FilesPython36lib
unpy.py", line 263, in run_path
    pkg_name=pkg_name, script_name=fname)
  File "C:Program FilesPython36lib
unpy.py", line 96, in _run_module_code
    mod_name, mod_spec, pkg_name, script_name)
  File "C:Program FilesPython36lib
unpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:UserschinaPycharmProjects
q3并发课堂windows_process.py", line 22, in <module>
    p.start()
  File "C:Program FilesPython36libmultiprocessingprocess.py", line 105, in start
    self._popen = self._Popen(self)
  File "C:Program FilesPython36libmultiprocessingcontext.py", line 223, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "C:Program FilesPython36libmultiprocessingcontext.py", line 322, in _Popen
    return Popen(process_obj)
  File "C:Program FilesPython36libmultiprocessingpopen_spawn_win32.py", line 33, in __init__
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "C:Program FilesPython36libmultiprocessingspawn.py", line 143, in get_preparation_data
    _check_not_importing_main()
  File "C:Program FilesPython36libmultiprocessingspawn.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__': #把进程创建放在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())
    # 默认所有进程结束后，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 子进程创建时，导入该模块输出main函数之前语句
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