Using chromedriver with selenium/python/ubuntu

I am trying to execute some tests using chromedriver and have tried using the following methods to start chromedriver.

driver = webdriver.Chrome('/usr/local/bin/chromedriver')

and

driver = webdriver.Chrome(executable_path='/usr/local/bin/chromedriver')

and

import os
from selenium import webdriver

chromedriver = "/usr/local/bin/chromedriver"
os.environ["webdriver.chrome.driver"] = chromedriver
driver = webdriver.Chrome(chromedriver)
driver.get("http://stackoverflow.com")

But none of these seems to help and the error is : selenium.common.exceptions.WebDriverException: Message: 'ChromeDriver executable needs to be available in the path.

I have checked multiple times and chromedriver is present in location /usr/local/bin.

Still my scripts are not working. Could any body pls help.

My google-chrome location is : /usr/bin/google-chrome