How to set trust-store used by Python's httplib?

Per default, Python's httplib.HTTPSConnection is using the system's trust store to validate a HTTPS certificate. (How) is it possible to set a different CA list to do the validation, replacing the default one?

I tried to set the SSLContext used by httplib (see below), but this only adds the certs from the given file; it does not stop the validator from loading more certs from the system's store if needed.

c = ssl.SSLContext(ssl.PROTOCOL_TLSv1)
c.load_verify_locations(cafile='mozillacerts.pem', capath=None)

conn = httplib.HTTPSConnection(domain, port, context=c)

To be clear: I want the validation to fail if it is not possible to validate the trust path with the certs in the given file.

(I am using Python 2.7.11 on Debian 8 for this.)

Solution

As it seems this is not really possible, we 'solved' the problem by using Python-requests, since it allows us to set the CA bundle.

requests.get('https://' + domain, verify='mozillacerts.pem')