from selenium import webdriver

browswer=webdriver.Firefox()

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 "<pyshell#2>", line 1, in <module>
browswer=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.

brew install geckodriver

sudo nano /etc/paths

/usr/local/bin
/usr/bin
/bin
/usr/sbin
/sbin
/usr/local/Cellar/geckodriver/0.21.0/bin/geckodriver