Earlier I used httplib module to add a header in the request. Now I am trying the same thing with the requests module.

This is the python request module I am using: http://pypi.python.org/pypi/requests

How can I add a header to request.post() and request.get(). Say I have to add foobar key in each request in the header.

From http://docs.python-requests.org/en/latest/user/quickstart/

url = 'https://api.github.com/some/endpoint'
payload = {'some': 'data'}
headers = {'content-type': 'application/json'}

r = requests.post(url, data=json.dumps(payload), headers=headers)

You just need to create a dict with your headers (key: value pairs where the key is the name of the header and the value is, well, the value of the pair) and pass that dict to the headers parameter on the .get or .post method.

So more specific to your question:

headers = {'foobar': 'raboof'}
requests.get('http://himom.com', headers=headers)

You can also do this to set a header for all future gets for the Session object, where x-test will be in all s.get() calls:

s = requests.Session()
s.auth = ('user', 'pass')
s.headers.update({'x-test': 'true'})

# both 'x-test' and 'x-test2' are sent
s.get('http://httpbin.org/headers', headers={'x-test2': 'true'})

from: http://docs.python-requests.org/en/latest/user/advanced/#session-objects