title: 'geckodriver' executable needs to be in PATH
body: <p>Mac OS user here. I'm trying to run a command in my python IDLE: </p>
  
  <pre><code>from selenium import webdriver
  browser = webdriver.Firefox()
  </code></pre>
  
  <p>and I get the following error message:</p>
  
  <pre><code>Traceback (most recent call last):
    File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/selenium/webdriver/common/service.py", line 76, in start
      stdin=PIPE)
    File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/subprocess.py", line 709, in __init__
      restore_signals, start_new_session)
    File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/subprocess.py", line 1344, in _execute_child
      raise child_exception_type(errno_num, err_msg, err_filename)
  FileNotFoundError: [Errno 2] No such file or directory: 'geckodriver': 'geckodriver'
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "&lt;pyshell#4&gt;", line 1, in &lt;module&gt;
      browser = webdriver.Firefox()
    File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/selenium/webdriver/firefox/webdriver.py", line 160, in __init__
      self.service.start()
    File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/selenium/webdriver/common/service.py", line 83, in start
      os.path.basename(self.path), self.start_error_message)
  selenium.common.exceptions.WebDriverException: Message: 'geckodriver' executable needs to be in PATH.
  </code></pre>
  
  <p>I've ran <code>brew install geckodriver</code>, and <code>which geckodriver</code> returns <code>/usr/local/bin/geckodriver</code> so I'm sure it's installed correctly. It seems like it's still not running properly however?</p>