I am working on a small project where I have to submit a form to a website.
The website is, however, using  onclick  event to submit the form (using javascript). 

How can the onclick event be simulated in python?
Which modules can be used? I have heard about selenium and mechanize modules. But, which module can be used or in case of both, which one is better?

I am new to web scraping and automation.So,it would be very helpful.
Thanks in advance.
Ideally you don't even need to clicks buttons in these kind of cases.
All you need is to see at what webservice does the form sends  request when clicked on submit button.
For that open your developer's control in the browser, Go to the Network tab and select 'preserve log'. Now submit the form manually and look for the first xhr GET/POST request sent. It would be POST request 90% of times. 
Now when you select that request in the request parameters it would show the values that you entered while submitting the form. Bingo!!
Now all you need to do is mimic this request with relevant request headers and parameters in your python code using requests. And Wooshh!!
Hope it helps..
How can I simulate onclick event in python?