我使用Metaclass创建Singleton类,它在多线程中工作良好,并且仅创建MySingleton类的一个实例,但是在多处理中,它总是创建新实例
import multiprocessing
class SingletonType(type):
# meta class for making a class singleton
def __call__(cls, *args, **kwargs):
try:
return cls.__instance
except AttributeError:
cls.__instance = super(SingletonType, cls).__call__(*args, **kwargs)
return cls.__instance
class MySingleton(object):
# singleton class
__metaclass__ = SingletonType
def __init__(*args,**kwargs):
print "init called"
def task():
# create singleton class instance
a = MySingleton()
# create two process
pro_1 = multiprocessing.Process(target=task)
pro_2 = multiprocessing.Process(target=task)
# start process
pro_1.start()
pro_2.start()
我的输出:
init called
init called
我需要MySingleton类的init方法仅被调用一次
最佳答案
您的每个子进程都运行自己的Python解释器实例,因此一个进程中的SingletonType
不会与另一进程中的共享其状态。这意味着仅存在于您的进程中的一个真正的单例几乎没有用,因为您将无法在其他进程中使用它:虽然可以manually share data between processes,但仅限于基本数据类型(例如字典和列表)。
无需依赖单例,只需在流程之间共享基础数据即可:
#!/usr/bin/env python3
import multiprocessing
import os
def log(s):
print('{}: {}'.format(os.getpid(), s))
class PseudoSingleton(object):
def __init__(*args,**kwargs):
if not shared_state:
log('Initializating shared state')
with shared_state_lock:
shared_state['x'] = 1
shared_state['y'] = 2
log('Shared state initialized')
else:
log('Shared state was already initalized: {}'.format(shared_state))
def task():
a = PseudoSingleton()
if __name__ == '__main__':
# We need the __main__ guard so that this part is only executed in
# the parent
log('Communication setup')
shared_state = multiprocessing.Manager().dict()
shared_state_lock = multiprocessing.Lock()
# create two process
log('Start child processes')
pro_1 = multiprocessing.Process(target=task)
pro_2 = multiprocessing.Process(target=task)
pro_1.start()
pro_2.start()
# Wait until processes have finished
# See https://stackoverflow.com/a/25456494/857390
log('Wait for children')
pro_1.join()
pro_2.join()
log('Done')
此打印
16194: Communication setup
16194: Start child processes
16194: Wait for children
16200: Initializating shared state
16200: Shared state initialized
16201: Shared state was already initalized: {'x': 1, 'y': 2}
16194: Done
但是,根据您的问题设置,使用其他进程间通信机制可能会有更好的解决方案。例如,
Queue
class通常非常有用。