我运行了以下代码

>>> from selenium import webdriver
>>> driver = webdriver.Firefox()

但出现了一个例外：

Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    driver = webdriver.Firefox()
  File "C:\Users\zoaoy\AppData\Local\Programs\Python\Python37\lib\site-packages\selenium\webdriver\firefox\webdriver.py", line 174, in __init__
    keep_alive=True)
  File "C:\Users\zoaoy\AppData\Local\Programs\Python\Python37\lib\site-packages\selenium\webdriver\remote\webdriver.py", line 157, in __init__
    self.start_session(capabilities, browser_profile)
  File "C:\Users\zoaoy\AppData\Local\Programs\Python\Python37\lib\site-packages\selenium\webdriver\remote\webdriver.py", line 252, in start_session
    response = self.execute(Command.NEW_SESSION, parameters)
  File "C:\Users\zoaoy\AppData\Local\Programs\Python\Python37\lib\site-packages\selenium\webdriver\remote\webdriver.py", line 321, in execute
    self.error_handler.check_response(response)
  File "C:\Users\zoaoy\AppData\Local\Programs\Python\Python37\lib\site-packages\selenium\webdriver\remote\errorhandler.py", line 242, in check_response
    raise exception_class(message, screen, stacktrace)
selenium.common.exceptions.SessionNotCreatedException: Message: Expected browser binary location, but unable to find binary in default location, no 'moz:firefoxOptions.binary' capability provided, and no binary flag set on the command line