python中的多处理-UnboundLocalError：赋值前引用了局部变量'data'

我正在尝试访问一个API，该API返回一组产品。由于执行速度很慢，所以我希望可以使用多重处理来使其更快。使用简单的for循环进行访问时，API可以完美运行。

这是我的代码：

from multiprocessing import Pool
from urllib2 import Request, urlopen, URLError
import json

def f(a):
    request = Request('API'+ str(a)) 
    try:
        response = urlopen(request)
        data = response.read()
    except URLError, e:
        print 'URL ERROR:', e
    s=json.loads(data)
    #count += len(s['Results'])
    #print count
    products=[]
    for i in range(len(s['Results'])):
        if (s['Results'][i]['IsSyndicated']==False):        
            try:
                products.append(int(s['Results'][i]['ProductId']))
            except ValueError as e:
                products.append(s['Results'][i]['ProductId'])
    return products

list=[0,100,200]

if __name__ == '__main__':
    p = Pool(4)
    result=p.map(f, list)
    print result

这是错误消息：

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\z080302\Desktop\WinPython-32bit-2.7.6.3\python-2.7.6\lib\site-packages\spyderlib\widgets\externalshell\sitecustomize.py", line 540, in runfile
    execfile(filename, namespace)
  File "C:/Users/z080302/Desktop/Python_Projects/mp_test.py", line 36, in <module>
    result=p.map(f, list)
  File "C:\Users\z080302\Desktop\WinPython-32bit-2.7.6.3\python-2.7.6\lib\multiprocessing\pool.py", line 250, in map
    return self.map_async(func, iterable, chunksize).get()
  File "C:\Users\z080302\Desktop\WinPython-32bit-2.7.6.3\python-2.7.6\lib\multiprocessing\pool.py", line 554, in get
    raise self._value
UnboundLocalError: local variable 'data' referenced before assignment

我在想，即使进行了多处理，该功能仍将按顺序执行。那我为什么要得到UnboundLocalError呢？