# 0x03 共享变量

## 共享变量

多进程中的每个进程与多线程中不同, 是不会共享变量的, 所使用的变量每个进程都会独自拷贝一份, 如果使用共享变量, 直接使用Python中的对象是不可行的:

```python
import os
from multiprocessing import Process

vip_list = []

def testFunc(cc):
    vip_list.append(cc)
    print('process id:', os.getpid())

if __name__ == '__main__':
    threads = []

    for ll in range(10):
        t = Process(target=testFunc, args=(ll,))
        t.daemon = True
        threads.append(t)

    for i in range(len(threads)):
        threads[i].start()

    for j in range(len(threads)):
        threads[j].join()

    print("------------------------")
    print('process id:', os.getpid())
    print(vip_list)
```

输出的结果为:

```
python aa.py
process id: 632
process id: 635
process id: 637
process id: 633
process id: 636
process id: 634
process id: 639
process id: 638
process id: 641
process id: 640
------------------------
process id: 619
[]
```

可以看到, 我们想让所有的进程把结果都存入到`vip_list`中, 但是每个进程都不会把结果存进去, 仍然是一个空列表. 这是因为`vip_list`列表不是一个共享变量, 不能被子进程使用.

可以使用`multiprocessing`包自带的共享变量类来实现. 其中的**Value**, **Array**以及**Manager**类都可以作为共享变量, 被多个进程共享使用.

* **Value**: 共享数值
* **Array**共享列表

```python
from multiprocessing import Process, Value, Array

def f(n, a):
    n.value = 3.1415927
    for i in range(len(a)):
        a[i] = -a[i]

if __name__ == '__main__':
    num = Value('d', 0.0)
    arr = Array('i', range(10))

    p = Process(target=f, args=(num, arr))
    p.start()
    p.join()

    print num.value
    print arr[:]
```

结果为:

```
3.1415927
[0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
```

* **Manager**: 支持多种形式, `list`, `dict`等Python对象以及`Value`, `Array`和`Queue`, `Event`, `Lock`等并行同步工具. 只需要调用`Manager`对象对应的属性.

  \`\`\`python import os from multiprocessing import Process, Manager, Lock

  lock = Lock() manager = Manager() sum = manager.Value('tmp', 0)

def testFunc(cc, lock): with lock: sum.value += cc

if **name** == '**main**': threads = \[]

```
  for ll in range(100):
      t = Process(target=testFunc, args=(1, lock))
      t.daemon = True
      threads.append(t)

  for i in range(len(threads)):
      threads[i].start()

  for j in range(len(threads)):
      threads[j].join()

  print("------------------------")
  print('process id:', os.getpid())
  print(sum.value)
```

\`\`\`

## 参考资料

[浅谈 python multiprocessing（多进程）下如何共享变量](https://my.oschina.net/leejun2005/blog/203148)


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