New submission from Paul Moore <p.f.mo...@gmail.com>:
If I run the following sample program using Python 3.7.3 (64 bit) for Windows,
it immediately fails, producing a massive traceback.
import multiprocessing
def f(n):
return n+1
with multiprocessing.Pool() as p:
for n in p.map(f, [1,2,3]):
print(n)
This is a part of the traceback produced - the whole traceback seemed to be
continually increasing, until I killed (some of) the python processes via task
manager:
Traceback (most recent call last):
File "<string>", line 1, in <module>
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\spawn.py",
line 105, in spawn_main
exitcode = _main(fd)
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\spawn.py",
line 114, in _main
prepare(preparation_data)
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\spawn.py",
line 225, in prepare
_fixup_main_from_path(data['init_main_from_path'])
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\spawn.py",
line 277, in _fixup_main_from_path
run_name="__mp_main__")
File "C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\runpy.py",
line 263, in run_path
pkg_name=pkg_name, script_name=fname)
File "C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\runpy.py",
line 96, in _run_module_code
Traceback (most recent call last):
File "<string>", line 1, in <module>
mod_name, mod_spec, pkg_name, script_name)
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\spawn.py",
line 105, in spawn_main
File "C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\runpy.py",
line 85, in _run_code
exitcode = _main(fd)
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\spawn.py",
line 114, in _main
exec(code, run_globals)
prepare(preparation_data)
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\spawn.py",
line 225, in prepare
File "C:\Work\Scratch\t.py", line 6, in <module>
_fixup_main_from_path(data['init_main_from_path'])
with multiprocessing.Pool() as p:
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\spawn.py",
line 277, in _fixup_main_from_path
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\context.py",
line 119, in Pool
run_name="__mp_main__")
File "C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\runpy.py",
line 263, in run_path
context=self.get_context())
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\pool.py",
line 176, in __init__
pkg_name=pkg_name, script_name=fname)
File "C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\runpy.py",
line 96, in _run_module_code
mod_name, mod_spec, pkg_name, script_name)
File "C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\runpy.py",
line 85, in _run_code
self._repopulate_pool()
exec(code, run_globals)
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\pool.py",
line 241, in _repopulate_pool
File "C:\Work\Scratch\t.py", line 6, in <module>
with multiprocessing.Pool() as p:
w.start()
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\context.py",
line 119, in Pool
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\process.py",
line 112, in start
context=self.get_context())
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\pool.py",
line 176, in __init__
self._popen = self._Popen(self)
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\context.py",
line 322, in _Popen
self._repopulate_pool()
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\pool.py",
line 241, in _repopulate_pool
return Popen(process_obj)
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\popen_spawn_win32.py",
line 46, in __init__
w.start()
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\process.py",
line 112, in start
prep_data = spawn.get_preparation_data(process_obj._name)
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\spawn.py",
line 143, in get_preparation_data
self._popen = self._Popen(self)
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\context.py",
line 322, in _Popen
return Popen(process_obj)
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\popen_spawn_win32.py",
line 46, in __init__
_check_not_importing_main()
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\spawn.py",
line 136, in _check_not_importing_main
prep_data = spawn.get_preparation_data(process_obj._name)
File
"C:\Users\pmoore\AppData\Local\Programs\Python\Python37\lib\multiprocessing\spawn.py",
line 143, in get_preparation_data
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
_check_not_importing_main()
in the main module:
if __name__ == '__main__':
freeze_support()
----------
components: Library (Lib), Windows
messages: 338870
nosy: paul.moore, steve.dower, tim.golden, zach.ware
priority: high
severity: normal
status: open
title: multiprocessing crashes in Python 3.7.3 on Windows
type: behavior
versions: Python 3.7
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue36435>
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