在python线程安全中修改类变量？

Question

我正在读这个问题(你不必阅读,因为我会复制那里的东西......我只是想告诉你我的灵感)......

所以,如果我有一个类来计算创建了多少个实例:

class Foo(object):
  instance_count = 0
  def __init__(self):
    Foo.instance_count += 1

我的问题是,如果我在多个线程中创建Foo对象,instance_count是否正确？类变量是否可以安全地从多个线程修改？

Answer 1

即使在CPython上也不是线程安全的.试试这个看看自己:

import threading

class Foo(object):
    instance_count = 0

def inc_by(n):
    for i in xrange(n):
        Foo.instance_count += 1

threads = [threading.Thread(target=inc_by, args=(100000,)) for thread_nr in xrange(100)]
for thread in threads: thread.start()
for thread in threads: thread.join()

print(Foo.instance_count) # Expected 10M for threadsafe ops, I get around 5M

原因是虽然INPLACE_ADD在GIL下是原子的,但属性仍然被加载并存储(参见dis.dis(Foo .__ init __)).使用锁定序列化对类变量的访问:

Foo.lock = threading.Lock()

def interlocked_inc(n):
    for i in xrange(n):
        with Foo.lock:
            Foo.instance_count += 1

threads = [threading.Thread(target=interlocked_inc, args=(100000,)) for thread_nr in xrange(100)]
for thread in threads: thread.start()
for thread in threads: thread.join()

print(Foo.instance_count)

Answer 2

不,它不是线程安全的.我几天前遇到过类似的问题,我选择通过装饰器实现锁定.好处是它使代码可读:

def threadsafe_function(fn):
    """decorator making sure that the decorated function is thread safe"""
    lock = threading.Lock()
    def new(*args, **kwargs):
        lock.acquire()
        try:
            r = fn(*args, **kwargs)
        except Exception as e:
            raise e
        finally:
            lock.release()
        return r
    return new

class X:
    var = 0

    @threadsafe_function     
    def inc_var(self):
        X.var += 1    
        return X.var