Can't run process from another Python .py file

Question

I am creating a class and when initializing it, I am calling (multiprocess) its function. When I am creating this class from the same .py file, everything works fine. But, when I am initializing this class from another .py file - it fails and throws an error... Why?

MyDummyClass.py:

import multiprocessing
class MyDummyClass:            
    def __init__(self):        
    
        print("I am in '__init__()'")
        
        if __name__ == 'MyDummyClass':
            
            p = multiprocessing.Process(target=self.foo())
            p.start()
            
        
    def foo(self):
        print("Hello from foo()")
        

example.py:

from MyDummyClass import MyDummyClass 
dummy = MyDummyClass()

When I run example.py, I am getting this error:

I am in '__init__()'
Hello from foo()
I am in '__init__()'
Hello from foo()

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Python\lib\multiprocessing\spawn.py", line 105, in spawn_main
    exitcode = _main(fd)
  File "C:\Python\lib\multiprocessing\spawn.py", line 114, in _main
    prepare(preparation_data)
  File "C:\Python\lib\multiprocessing\spawn.py", line 225, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "C:\Python\lib\multiprocessing\spawn.py", line 277, in _fixup_main_from_path
    run_name="__mp_main__")
  File "C:\Python\lib\runpy.py", line 263, in run_path
    pkg_name=pkg_name, script_name=fname)
  File "C:\Python\lib\runpy.py", line 96, in _run_module_code
    mod_name, mod_spec, pkg_name, script_name)
  File "C:\Python\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "E:\Dropbox\Docs\Business\_NZTrackAlerts\Website\Current dev\NZtracker\cgi-bin\example1.py", line 2, in <module>
    dummy = MyDummyClass()
  File "...path to my file...\MyDummyClass.py", line 13, in __init__
    p.start()
  File "C:\Python\lib\multiprocessing\process.py", line 105, in start
    self._popen = self._Popen(self)
  File "C:\Python\lib\multiprocessing\context.py", line 223, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "C:\Python\lib\multiprocessing\context.py", line 322, in _Popen
    return Popen(process_obj)
  File "C:\Python\lib\multiprocessing\popen_spawn_win32.py", line 33, in __init__
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "C:\Python\lib\multiprocessing\spawn.py", line 143, in get_preparation_data
    _check_not_importing_main()
  File "C:\Python\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.

I can't understand (and not from other posts as well) how to fix it.

Answer 1

This has to do with the way that python actually executes scripts. The error that it's throwing essentially says that you're trying to start a new process when python isn't ready yet. This is because you're calling dummy = MyDummyClass() in the main script without giving python a chance to fully initialize. If you instead have example.py like this:

from MyDummyClass import MyDummyClass 
if __name__ == "__main__":
    dummy = MyDummyClass()

This will produce your desired output:

C:\Python Scripts>python example.py
I am in '__init__()'
Hello from foo()

The if __name__ == "__main__" block tells python "only execute this if the script is being run as the main script (i.e. python example.py), and this will force python to initialize everything properly before it runs that block.

Apologies if this answer doesn't describe enough what's going on in the back end of python as it's doing the initializing, I don't know a ton about it myself :)