Recipe Multiprocessing

Hartmut Goebel edited this page Nov 26, 2017 · 7 revisions

Required Changes to your code

When using the multiprocessing module, you must call


straight after the if __name__ == '__main__': line of the main module.

Please read the Python library manual about multiprocessing.freeze_support for more information.

Additional code for PyInstaller < 3.3 and Windows

As of PyInstaller 3.3. adding this code is not longer required, it is already added by a run-time hook.

On Windows, Multiprocessing code fails when using a --onefile executable. This problem is specific to Windows, which does not support spawn(). It does not occur when using the default (--onedir) mode neither does it occur on other (Posix) platforms like all flavors of Unix and Mac OS X.

For using python module _multiprocess_ on Windows, you need to extend your multiprocessing code as shown below. See this thread about the background and ticket for more information.

This recipe requires PyInstaller 3.0 < 3.3.

import os
import sys

# Module multiprocessing is organized differently in Python 3.4+
    # Python 3.4+
    if sys.platform.startswith('win'):
        import multiprocessing.popen_spawn_win32 as forking
        import multiprocessing.popen_fork as forking
except ImportError:
    import multiprocessing.forking as forking

if sys.platform.startswith('win'):
    # First define a modified version of Popen.
    class _Popen(forking.Popen):
        def __init__(self, *args, **kw):
            if hasattr(sys, 'frozen'):
                # We have to set original _MEIPASS2 value from sys._MEIPASS
                # to get --onefile mode working.
                os.putenv('_MEIPASS2', sys._MEIPASS)
                super(_Popen, self).__init__(*args, **kw)
                if hasattr(sys, 'frozen'):
                    # On some platforms (e.g. AIX) 'os.unsetenv()' is not
                    # available. In those cases we cannot delete the variable
                    # but only set it to the empty string. The bootloader
                    # can handle this case.
                    if hasattr(os, 'unsetenv'):
                        os.putenv('_MEIPASS2', '')

    # Second override 'Popen' class with our modified version.
    forking.Popen = _Popen

Example for testing multiprocessing:

import multiprocessing

class SendeventProcess(multiprocessing.Process):
    def __init__(self, resultQueue):
        self.resultQueue = resultQueue

    def run(self):
        print 'SendeventProcess'
        self.resultQueue.put((1, 2))
        print 'SendeventProcess'

if __name__ == '__main__':
    # On Windows calling this function is necessary.
    # On Linux/OSX it does nothing.
    print 'main'
    resultQueue = multiprocessing.Queue()
    print 'main'

Console output of this code snippet should be similar to

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