1.用法注释
1.1 Process类
import multiprocessing as mp
def foo(q):
q.put('hello')
if __name__ == '__main__':
mp.set_start_method('spawn')
q = mp.Queue()
p = mp.Process(target=foo, args=(q,))
p.start()
print(q.get())
p.join()
这里值得注意的是这个Queue，是可以用在多进程环境下的阻塞队列
我们可以看一下Process类的实现
class Process(object):
'''
Process objects represent activity that is run in a separate process
The class is analagous to `threading.Thread`
'''
_Popen = None
def __init__(self, group=None, target=None, name=None, args=(), kwargs={}):
assert group is None, 'group argument must be None for now'
count = _current_process._counter.next()
self._identity = _current_process._identity + (count,)
self._authkey = _current_process._authkey
self._daemonic = _current_process._daemonic
self._tempdir = _current_process._tempdir
self._parent_pid = os.getpid()
self._popen = None
self._target = target
self._args = tuple(args)
self._kwargs = dict(kwargs)
self._name = name or type(self).__name__ + '-' + \
':'.join(str(i) for i in self._identity)
def run(self):
'''
Method to be run in sub-process; can be overridden in sub-class
'''
if self._target:
self._target(*self._args, **self._kwargs)
def start(self):
'''
Start child process
'''
assert self._popen is None, 'cannot start a process twice'
assert self._parent_pid == os.getpid(), \
'can only start a process object created by current process'
assert not _current_process._daemonic, \
'daemonic processes are not allowed to have children'
_cleanup()
if self._Popen is not None:
Popen = self._Popen
else:
from .forking import Popen
self._popen = Popen(self)
_current_process._children.add(self)
def terminate(self):
'''
Terminate process; sends SIGTERM signal or uses TerminateProcess()
'''
self._popen.terminate()
def join(self, timeout=None):
'''
Wait until child process terminates
'''
assert self._parent_pid == os.getpid(), 'can only join a child process'
assert self._popen is not None, 'can only join a started process'
res = self._popen.wait(timeout)
if res is not None:
_current_process._children.discard(self)
def is_alive(self):
'''
Return whether process is alive
'''
if self is _current_process:
return True
assert self._parent_pid == os.getpid(), 'can only test a child process'
if self._popen is None:
return False
self._popen.poll()
return self._popen.returncode is None
@property
def name(self):
return self._name
@name.setter
def name(self, name):
assert isinstance(name, basestring), 'name must be a string'
self._name = name
@property
def daemon(self):
'''
Return whether process is a daemon
'''
return self._daemonic
@daemon.setter
def daemon(self, daemonic):
'''
Set whether process is a daemon
'''
assert self._popen is None, 'process has already started'
self._daemonic = daemonic
@property
def authkey(self):
return self._authkey
@authkey.setter
def authkey(self, authkey):
'''
Set authorization key of process
'''
self._authkey = AuthenticationString(authkey)
@property
def exitcode(self):
'''
Return exit code of process or `None` if it has yet to stop
'''
if self._popen is None:
return self._popen
return self._popen.poll()
@property
def ident(self):
'''
Return identifier (PID) of process or `None` if it has yet to start
'''
if self is _current_process:
return os.getpid()
else:
return self._popen and self._popen.pid
pid = ident
def __repr__(self):
if self is _current_process:
status = 'started'
elif self._parent_pid != os.getpid():
status = 'unknown'
elif self._popen is None:
status = 'initial'
else:
if self._popen.poll() is not None:
status = self.exitcode
else:
status = 'started'
if type(status) is int:
if status == 0:
status = 'stopped'
else:
status = 'stopped[%s]' % _exitcode_to_name.get(status, status)
return '<%s(%s, %s%s)>' % (type(self).__name__, self._name,
status, self._daemonic and ' daemon' or '')
##
def _bootstrap(self):
from . import util
global _current_process
try:
self._children = set()
self._counter = itertools.count(1)
try:
sys.stdin.close()
sys.stdin = open(os.devnull)
except (OSError, ValueError):
pass
_current_process = self
util._finalizer_registry.clear()
util._run_after_forkers()
util.info('child process calling self.run()')
try:
self.run()
exitcode = 0
finally:
util._exit_function()
except SystemExit, e:
if not e.args:
exitcode = 1
elif isinstance(e.args[0], int):
exitcode = e.args[0]
else:
sys.stderr.write(str(e.args[0]) + '\n')
sys.stderr.flush()
exitcode = 1
except:
exitcode = 1
import traceback
sys.stderr.write('Process %s:\n' % self.name)
sys.stderr.flush()
traceback.print_exc()
util.info('process exiting with exitcode %d' % exitcode)
return exitcode
这里是参考threading的做法封装的一些接口，只不过join函数是基于waitpid实现的，而线程里面的join是调用的pthreadd的pthread_join系统调用。从接口层面值得注意的是run和start函数，可以看到run方法很简单就是执行了一个函数，但是一般我们对于process object不会直接去调用run函数（这样就等于在当前进程执行了一个函数，没有意义），我们会调用start函数，由start函数负责在另一个进程里调度run函数，当然这里为了维持Process object的定义简单化，固定一个Process对象只能执行一次。换句话说，一个process对象的完整生命周期就是从start开始，到执行target函数完成退出或者中间被terminate。这对于子过程调用而言是十分重要的简化，大多数时候我们就是单纯的希望将某个耗时任务放到一个子进程里面进行，从而并行化计算任务，提高CPU占用率。
另一个值得注意的class是context对象
import os
import sys
import threading
from . import process
from . import reduction
__all__ = [] # things are copied from here to __init__.py
#
# Exceptions
#
class ProcessError(Exception):
pass
class BufferTooShort(ProcessError):
pass
class TimeoutError(ProcessError):
pass
class AuthenticationError(ProcessError):
pass
#
# Base type for contexts
#
class BaseContext(object):
ProcessError = ProcessError
BufferTooShort = BufferTooShort
TimeoutError = TimeoutError
AuthenticationError = AuthenticationError
current_process = staticmethod(process.current_process)
active_children = staticmethod(process.active_children)
def cpu_count(self):
'''Returns the number of CPUs in the system'''
num = os.cpu_count()
if num is None:
raise NotImplementedError('cannot determine number of cpus')
else:
return num
def Manager(self):
'''Returns a manager associated with a running server process
The managers methods such as `Lock()`, `Condition()` and `Queue()`
can be used to create shared objects.
'''
from .managers import SyncManager
m = SyncManager(ctx=self.get_context())
m.start()
return m
def Pipe(self, duplex=True):
'''Returns two connection object connected by a pipe'''
from .connection import Pipe
return Pipe(duplex)
def Lock(self):
'''Returns a non-recursive lock object'''
from .synchronize import Lock
return Lock(ctx=self.get_context())
def RLock(self):
'''Returns a recursive lock object'''
from .synchronize import RLock
return RLock(ctx=self.get_context())
def Condition(self, lock=None):
'''Returns a condition object'''
from .synchronize import Condition
return Condition(lock, ctx=self.get_context())
def Semaphore(self, value=1):
'''Returns a semaphore object'''
from .synchronize import Semaphore
return Semaphore(value, ctx=self.get_context())
def BoundedSemaphore(self, value=1):
'''Returns a bounded semaphore object'''
from .synchronize import BoundedSemaphore
return BoundedSemaphore(value, ctx=self.get_context())
def Event(self):
'''Returns an event object'''
from .synchronize import Event
return Event(ctx=self.get_context())
def Barrier(self, parties, action=None, timeout=None):
'''Returns a barrier object'''
from .synchronize import Barrier
return Barrier(parties, action, timeout, ctx=self.get_context())
def Queue(self, maxsize=0):
'''Returns a queue object'''
from .queues import Queue
return Queue(maxsize, ctx=self.get_context())
def JoinableQueue(self, maxsize=0):
'''Returns a queue object'''
from .queues import JoinableQueue
return JoinableQueue(maxsize, ctx=self.get_context())
def SimpleQueue(self):
'''Returns a queue object'''
from .queues import SimpleQueue
return SimpleQueue(ctx=self.get_context())
def Pool(self, processes=None, initializer=None, initargs=(),
maxtasksperchild=None):
'''Returns a process pool object'''
from .pool import Pool
return Pool(processes, initializer, initargs, maxtasksperchild,
context=self.get_context())
def RawValue(self, typecode_or_type, *args):
'''Returns a shared object'''
from .sharedctypes import RawValue
return RawValue(typecode_or_type, *args)
def RawArray(self, typecode_or_type, size_or_initializer):
'''Returns a shared array'''
from .sharedctypes import RawArray
return RawArray(typecode_or_type, size_or_initializer)
def Value(self, typecode_or_type, *args, lock=True):
'''Returns a synchronized shared object'''
from .sharedctypes import Value
return Value(typecode_or_type, *args, lock=lock,
ctx=self.get_context())
def Array(self, typecode_or_type, size_or_initializer, *, lock=True):
'''Returns a synchronized shared array'''
from .sharedctypes import Array
return Array(typecode_or_type, size_or_initializer, lock=lock,
ctx=self.get_context())
def freeze_support(self):
'''Check whether this is a fake forked process in a frozen executable.
If so then run code specified by commandline and exit.
'''
if sys.platform == 'win32' and getattr(sys, 'frozen', False):
from .spawn import freeze_support
freeze_support()
def get_logger(self):
'''Return package logger -- if it does not already exist then
it is created.
'''
from .util import get_logger
return get_logger()
def log_to_stderr(self, level=None):
'''Turn on logging and add a handler which prints to stderr'''
from .util import log_to_stderr
return log_to_stderr(level)
def allow_connection_pickling(self):
'''Install support for sending connections and sockets
between processes
'''
# This is undocumented. In previous versions of multiprocessing
# its only effect was to make socket objects inheritable on Windows.
from . import connection
def set_executable(self, executable):
'''Sets the path to a python.exe or pythonw.exe binary used to run
child processes instead of sys.executable when using the 'spawn'
start method. Useful for people embedding Python.
'''
from .spawn import set_executable
set_executable(executable)
def set_forkserver_preload(self, module_names):
'''Set list of module names to try to load in forkserver process.
This is really just a hint.
'''
from .forkserver import set_forkserver_preload
set_forkserver_preload(module_names)
def get_context(self, method=None):
if method is None:
return self
try:
ctx = _concrete_contexts[method]
except KeyError:
raise ValueError('cannot find context for %r' % method) from None
ctx._check_available()
return ctx
def get_start_method(self, allow_none=False):
return self._name
def set_start_method(self, method, force=False):
raise ValueError('cannot set start method of concrete context')
@property
def reducer(self):
'''Controls how objects will be reduced to a form that can be
shared with other processes.'''
return globals().get('reduction')
@reducer.setter
def reducer(self, reduction):
globals()['reduction'] = reduction
def _check_available(self):
pass
#
# Type of default context -- underlying context can be set at most once
#
class Process(process.BaseProcess):
_start_method = None
@staticmethod
def _Popen(process_obj):
return _default_context.get_context().Process._Popen(process_obj)
class DefaultContext(BaseContext):
Process = Process
def __init__(self, context):
self._default_context = context
self._actual_context = None
def get_context(self, method=None):
if method is None:
if self._actual_context is None:
self._actual_context = self._default_context
return self._actual_context
else:
return super().get_context(method)
def set_start_method(self, method, force=False):
if self._actual_context is not None and not force:
raise RuntimeError('context has already been set')
if method is None and force:
self._actual_context = None
return
self._actual_context = self.get_context(method)
def get_start_method(self, allow_none=False):
if self._actual_context is None:
if allow_none:
return None
self._actual_context = self._default_context
return self._actual_context._name
def get_all_start_methods(self):
if sys.platform == 'win32':
return ['spawn']
else:
if reduction.HAVE_SEND_HANDLE:
return ['fork', 'spawn', 'forkserver']
else:
return ['fork', 'spawn']
DefaultContext.__all__ = [x for x in dir(DefaultContext) if x[0] != '_']
#
# Context types for fixed start method
#
if sys.platform != 'win32':
class ForkProcess(process.BaseProcess):
_start_method = 'fork'
@staticmethod
def _Popen(process_obj):
from .popen_fork import Popen
return Popen(process_obj)
class SpawnProcess(process.BaseProcess):
_start_method = 'spawn'
@staticmethod
def _Popen(process_obj):
from .popen_spawn_posix import Popen
return Popen(process_obj)
class ForkServerProcess(process.BaseProcess):
_start_method = 'forkserver'
@staticmethod
def _Popen(process_obj):
from .popen_forkserver import Popen
return Popen(process_obj)
class ForkContext(BaseContext):
_name = 'fork'
Process = ForkProcess
class SpawnContext(BaseContext):
_name = 'spawn'
Process = SpawnProcess
class ForkServerContext(BaseContext):
_name = 'forkserver'
Process = ForkServerProcess
def _check_available(self):
if not reduction.HAVE_SEND_HANDLE:
raise ValueError('forkserver start method not available')
_concrete_contexts = {
'fork': ForkContext(),
'spawn': SpawnContext(),
'forkserver': ForkServerContext(),
}
_default_context = DefaultContext(_concrete_contexts['fork'])
else:
class SpawnProcess(process.BaseProcess):
_start_method = 'spawn'
@staticmethod
def _Popen(process_obj):
from .popen_spawn_win32 import Popen
return Popen(process_obj)
class SpawnContext(BaseContext):
_name = 'spawn'
Process = SpawnProcess
_concrete_contexts = {
'spawn': SpawnContext(),
}
_default_context = DefaultContext(_concrete_contexts['spawn'])
#
# Force the start method
#
def _force_start_method(method):
_default_context._actual_context = _concrete_contexts[method]
#
# Check that the current thread is spawning a child process
#
_tls = threading.local()
def get_spawning_popen():
return getattr(_tls, 'spawning_popen', None)
def set_spawning_popen(popen):
_tls.spawning_popen = popen
def assert_spawning(obj):
if get_spawning_popen() is None:
raise RuntimeError(
'%s objects should only be shared between processes'
' through inheritance' % type(obj).__name__
)
从外部接口来看，Python的多进程库几乎是想复刻多线程的使用体验，如果没有特殊需求的话，一般只要按这种理念即可很好的使用多进程编程。
