当我使用Python使用Selenium执行此代码时：

from selenium import webdriver
from selenium.webdriver.common.by import By
import time
driver = webdriver.Chrome(executable_path=r'/Users/qa/Documents/Python/chromedriver')

发生错误：

   Traceback (most recent call last):
  File "/Users/qa/Documents/Python/try.py", line 4, in <module>
    driver = webdriver.Chrome(executable_path=r'/Users/qa/Documents/Python/chromedriver')
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/selenium/webdriver/chrome/webdriver.py", line 81, in __init__
    desired_capabilities=desired_capabilities)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/selenium/webdriver/remote/webdriver.py", line 157, in __init__
    self.start_session(capabilities, browser_profile)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/selenium/webdriver/remote/webdriver.py", line 252, in start_session
    response = self.execute(Command.NEW_SESSION, parameters)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/selenium/webdriver/remote/webdriver.py", line 321, in execute
    self.error_handler.check_response(response)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/selenium/webdriver/remote/errorhandler.py", line 242, in check_response
    raise exception_class(message, screen, stacktrace)
selenium.common.exceptions.SessionNotCreatedException: Message: session not created
from disconnected: unable to connect to renderer
  (Session info: chrome=71.0.3578.98)
  (Driver info: chromedriver=2.44.609545 (c2f88692e98ce7233d2df7c724465ecacfe74df5),platform=Mac OS X 10.13.6 x86_64)

有人能帮我吗？谢谢。