Python：The “freeze_support()“ line can be omitted if the program is not going to be frozen

该异常在windows执行python多线程时出现
完整的报错信息

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\spawn.py", line 105, in spawn_main
    exitcode = _main(fd)
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\spawn.py", line 114, in _main
    prepare(preparation_data)
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\spawn.py", line 225, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\spawn.py", line 277, in _fixup_main_from_path
    run_name="__mp_main__")
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\runpy.py", line 263, in run_path
Traceback (most recent call last):
  File "<string>", line 1, in <module>
    pkg_name=pkg_name, script_name=fname)
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\runpy.py", line 96, in _run_module_code
    mod_name, mod_spec, pkg_name, script_name)
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\runpy.py", line 85, in _run_code
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\spawn.py", line 105, in spawn_main
    exec(code, run_globals)
  File "D:\program\PythonProgram\test02.py", line 20, in <module>
    process1.start()
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\process.py", line 112, in start
    exitcode = _main(fd)
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\spawn.py", line 114, in _main
    self._popen = self._Popen(self)
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\context.py", line 223, in _Popen
    prepare(preparation_data)
    return _default_context.get_context().Process._Popen(process_obj)  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\spawn.py", line 225, in prepare

  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\context.py", line 322, in _Popen
    return Popen(process_obj)
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\popen_spawn_win32.py", line 46, in __init__
    _fixup_main_from_path(data['init_main_from_path'])
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\spawn.py", line 277, in _fixup_main_from_path
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\spawn.py", line 143, in get_preparation_data
    _check_not_importing_main()
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\spawn.py", line 136, in _check_not_importing_main
    run_name="__mp_main__")
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\runpy.py", line 263, in run_path
    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:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.
    pkg_name=pkg_name, script_name=fname)
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\runpy.py", line 96, in _run_module_code
    mod_name, mod_spec, pkg_name, script_name)
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "D:\program\PythonProgram\test02.py", line 20, in <module>
    process1.start()
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\process.py", line 112, in start
    self._popen = self._Popen(self)
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\context.py", line 223, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\context.py", line 322, in _Popen
    return Popen(process_obj)
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\popen_spawn_win32.py", line 46, in __init__
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\lib\multiprocessing\spawn.py", line 143, in get_preparation_data
    _check_not_importing_main()
  File "D:\Program Files (x86)\Microsoft Visual Studio\Shared\Python37_64\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:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.

解决方法：
在main中加入

if __name__ == '__main__':
	#你的代码

原因：
start方法产生了一个新的python，但在进程开始之前必须要导入multiprocessing，如果不用if保护创建代码则子进程会重新执行父进程的代码而产生新的进程(windows平台不支持pyhon中的一项默认设置，详:https://mxnet.apache.org/versions/1.2.1/_modules/mxnet/gluon/data/dataloader.html)，直到程序崩溃