Submitting to a web form using python

I have seen questions like this asked many many times but none are helpful

Im trying to submit data to a form on the web ive tried requests, and urllib and none have worked

for example here is code that should search for the [python] tag on SO:

import urllib
import urllib2

url = 'http://stackoverflow.com/'

# Prepare the data
values = {'q' : '[python]'}
data = urllib.urlencode(values)

# Send HTTP POST request
req = urllib2.Request(url, data)
response = urllib2.urlopen(req)

html = response.read()

# Print the result
print html

yet when i run it i get the html soure of the home page

here is an example of using requests:

import requests

data= {
    'q': '[python]'
    }
r = requests.get('http://stackoverflow.com', data=data)

print r.text

same result! i dont understand why these methods arent working i've tried them on various sites with no success so if anyone has successfully done this please show me how!

Thanks so much!