Selenium with Python- Message: 'operadriver' executable needs to be in PATH

for checking whether a website loads in opera using selenium with python, using the code

def test_opera_compatability(self):
    driver = webdriver.Opera("functional_tests/operadriver")
    driver.get("https://www.google.com/")
    driver.quit()

It returns the following error

Message: 'operadriver' executable needs to be in PATH.

similar code for chrome works as intended, which is like this

def test_chrome_compatability(self):
    driver = webdriver.Chrome('functional_tests/chromedriver')
    driver.get("https://www.google.com/")
    driver.quit()