Question

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()

Answer 1

这是我的答案，因为我没有在前面的答案中看到结论。

我开始想知道同样的事情并尝试了一个与其他答案类似的测试程序，我的结论是他们确实比程序结束时更早得到GCed，这意味着，这些引用可以被确定为垃圾一旦线程本身死亡。

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：

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

正如您所看到的，删除似乎一旦线程死亡就会发生。

Answer 2

Mark几乎是正确的 - 基本上“mydata”将保存对其中所有TL变量的引用，无论它们是从哪个线程创建的。就是......：

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!"

可发出：

t created
t started
x got deleted!
mydata deleted
t joined
Done!

gc在CPython中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!"

仍然看到......：

t created
x got created!
t started
x got deleted!
mydata deleted
t joined
Done!

Answer 3

谢谢！在CPython 2.5和2.6下，Mark的程序似乎表现不同：

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!"

Emits（在Ubuntu 8.04 i386下）：

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!

和

Python 2.6.2 (r262:71600 Sep 19 2009 17:24:20)
t created
t started
x got deleted!
mydata deleted
t joined
Done!

Answer 4

通过对程序进行一些简单的更改并在线程的每个步骤之后强制进行垃圾回收，似乎在程序完成之前无法收集foo - 换句话说，之后< / em>线程超出范围。

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 created t started t joined Done! x got deleted!

Python中线程局部值的生命周期是什么时候？

4 个答案: