I had Selenium's chromedriver in the same directory as the python script files. The chromedriver is in the current directory as seen from the script file, so it worked below.
python
driver = webdriver.Chrome()
However, one day, when I updated my Mac OS, I got an error saying that the chromedriver could not be found (passed through the path).
As a workaround, I modified it below and it worked fine.
python
driver = webdriver.Chrome(executable_path='./chromedriver')