Exception running the examples on README

[riyadparvez]

I am on Mac M1 chip. I am using python 3.9.

I've tried the example listed on README page:

from pebble import concurrent
from concurrent.futures import TimeoutError

@concurrent.process(timeout=10)
def function(foo, bar=0):
    return foo + bar

future = function(1, bar=2)

try:
    result = future.result()  # blocks until results are ready
except TimeoutError as error:
    print("Function took longer than %d seconds" % error.args[1])
except Exception as error:
    print("Function raised %s" % error)
    print(error.traceback)  # traceback of the function

Here's the stack trace

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 125, in _main
    prepare(preparation_data)
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 236, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 287, in _fixup_main_from_path
    main_content = runpy.run_path(main_path,
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/runpy.py", line 268, in run_path
    return _run_module_code(code, init_globals, run_name,
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/runpy.py", line 97, in _run_module_code
    _run_code(code, mod_globals, init_globals,
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/Users/riyad/dev-src/chaos-theory/python-test/pebble-test.py", line 8, in <module>
    future = function(1, bar=2)
  File "/opt/homebrew/lib/python3.9/site-packages/pebble/concurrent/process.py", line 96, in wrapper
    worker = launch_process(
  File "/opt/homebrew/lib/python3.9/site-packages/pebble/common.py", line 141, in launch_process
    process.start()
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py", line 42, in _launch
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 154, in get_preparation_data
    _check_not_importing_main()
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 134, in _check_not_importing_main
    raise RuntimeError('''
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.
Function raised Abnormal termination
Traceback (most recent call last):
  File "/Users/riyad/dev-src/chaos-theory/python-test/pebble-test.py", line 11, in <module>
    result = future.result()  # blocks until results are ready
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/concurrent/futures/_base.py", line 446, in result
    return self.__get_result()
  File "/opt/homebrew/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/concurrent/futures/_base.py", line 391, in __get_result
    raise self._exception
pebble.common.ProcessExpired: Abnormal termination

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/riyad/dev-src/chaos-theory/python-test/pebble-test.py", line 16, in <module>
    print(error.traceback)  # traceback of the function
AttributeError: 'ProcessExpired' object has no attribute 'traceback'

[noxdafox]

Hello,

the exception is explaining the problem.

        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.

As per the documentation, pebble is affected by the same limitations of the multiprocessing library. In particular, on OSes such as MAC and Windows, the if __name__ == '__main__': guard it's required to be placed in your code: https://docs.python.org/3/library/multiprocessing.html#the-spawn-and-forkserver-start-methods