Python pool.map（）列表索引必须是整数，而不是str

Question

我有一个这样的示例代码：

from selenium import webdriver
from multiprocessing import Pool

url = ['https://www.google.com','https://www.facebook.com']
driver = webdriver.Firefox()

def browser(x):
    driver.get(url[x])
if __name__ == "__main__":
    pool = Pool(2)
    pool.map(browser,url)

收到错误：

Traceback (most recent call last):
  File "test.py", line 12, in <module>
    pool.map(browser,url)
  File "C:\Python27\lib\multiprocessing\pool.py", line 251, in map
    return self.map_async(func, iterable, chunksize).get()
  File "C:\Python27\lib\multiprocessing\pool.py", line 567, in get
    raise self._value
TypeError: list indices must be integers, not str

我做错了吗？