import threading
mydata = threading.local()
def run():
# When will the garbage collector be able to destroy the object created
# here? After the thread exits from ``run()``? After ``join()`` is called?
# Or will it survive the thread in which it was created, and live until
# ``mydata`` is garbage-collected?
mydata.foo = object()
t = threading.Thread(target=run)
t.start()
t.join()
Python中线程本地值的生命周期是多久？
13
- Carlos Valiente
4个回答
10
这是我的答案，因为我没有在之前的回答中看到结论。
我开始思考同样的问题，并尝试了一个类似于其他答案中的测试程序。我的结论是，它们比程序结束时更早地被垃圾收集，这意味着一旦线程本身死亡，这些引用就可以被确定为垃圾。
import time
import threading
import gc
data = threading.local()
class Resource(object):
def __init__(self):
self.name = threading.currentThread().name
print 'create: %s' % self.name
def __del__(self):
print 'delete: %s' % self.name
def access_thlocal():
data.key = Resource()
for i in range(0, 10):
threading.Thread(target=access_thlocal).start()
time.sleep(1)
print "Triggering GC"
gc.collect()
time.sleep(1)
输出结果：
create: Thread-1
create: Thread-2
delete: Thread-1
create: Thread-3
delete: Thread-2
create: Thread-4
delete: Thread-3
create: Thread-5
delete: Thread-4
create: Thread-6
delete: Thread-5
create: Thread-7
delete: Thread-6
create: Thread-8
delete: Thread-7
create: Thread-9
delete: Thread-8
create: Thread-10
delete: Thread-9
Triggering GC
delete: Thread-10
你可以看到，删除似乎是在线程终止时立即发生的。
- haridsv
1
1这似乎仍然是CPython 3.6中的当前行为。 - ash
3
Mark几乎说对了--实际上，“mydata”将持有其中所有TL变量的引用，无论它们是从哪个线程创建的。例如：
import threading
import gc
mydata = threading.local()
class x:
def __del__(self):
print "x got deleted!"
def run():
mydata.foo = x()
t = threading.Thread(target=run)
print "t created"
gc.collect()
t.start()
print "t started"
gc.collect()
del mydata
print "mydata deleted"
gc.collect()
t.join()
print "t joined"
gc.collect()
print "Done!"
发出：
t created
t started
x got deleted!
mydata deleted
t joined
Done!
在CPython中，gc实际上没有任何作用，因此您可以将代码简化为：
import threading
mydata = threading.local()
class x:
def __init__(self):
print "x got created!"
def __del__(self):
print "x got deleted!"
def run():
mydata.foo = x()
t = threading.Thread(target=run)
print "t created"
t.start()
print "t started"
del mydata
print "mydata deleted"
t.join()
print "t joined"
print "Done!"
并且仍然可以看到...
t created
x got created!
t started
x got deleted!
mydata deleted
t joined
Done!
- Alex Martelli
1
谢谢！看起来Mark的程序在CPython 2.5和2.6下表现不同：
import threading
import gc
import platform
print "Python %s (%s)" % (platform.python_version(), " ".join(platform.python_build()))
mydata = threading.local()
class x:
def __del__(self):
print "x got deleted!"
def run():
mydata.foo = x()
t = threading.Thread(target=run)
print "t created"
gc.collect()
t.start()
print "t started"
gc.collect()
del mydata
print "mydata deleted"
gc.collect()
t.join()
print "t joined"
gc.collect()
print "Done!"
在 Ubuntu 8.04 i386 下发出：
Python 2.5.2 (r252:60911 Jul 31 2008 19:40:22)
t created
t started
mydata deleted
x got deleted!
Exception in thread Thread-1:
Traceback (most recent call last):
File "/usr/lib/python2.5/threading.py", line 486, in __bootstrap_inner
self.run()
File "/usr/lib/python2.5/threading.py", line 446, in run
self.__target(*self.__args, **self.__kwargs)
File "./x.py", line 14, in run
mydata.foo = x()
NameError: global name 'mydata' is not defined
t joined
Done!
并且：
Python 2.6.2 (r262:71600 Sep 19 2009 17:24:20)
t created
t started
x got deleted!
mydata deleted
t joined
Done!
- Carlos Valiente
0
通过对程序进行一些简单的更改，并在每个线程步骤之后强制进行垃圾回收，似乎无法在程序完成之前收集
foo - 换句话说，在线程超出范围之后。import threading
import gc
mydata = threading.local()
class x:
def __del__(self):
print "x got deleted!"
def run():
mydata.foo = x()
t = threading.Thread(target=run)
print "t created"
gc.collect()
t.start()
print "t started"
gc.collect()
t.join()
print "t joined"
gc.collect()
print "Done!"
输出（使用Python 2.6，Windows）：
>C:\temp\py\t.py t已创建 t已启动 t已加入 完成！ x已被删除！
- Mark Rushakoff
1
好的分析，但是遗漏的部分是 mydata 的消失是你所需要的 -- 我需要打开一个新的答案来展示格式化的代码，但是要点与你的差不多。 - Alex Martelli
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