Retrieves a password from the keychain based on the environment and
    configuration parameter pair.

    If this fails, None is returned.

def password_get(username=None):
    """
    Retrieves a password from the keychain based on the environment and
    configuration parameter pair.

    If this fails, None is returned.
    """
    password = keyring.get_password('supernova', username)
    if password is None:
        split_username = tuple(username.split(':'))
        msg = ("Couldn't find a credential for {0}:{1}. You need to set one "
               "with: supernova-keyring -s {0} {1}").format(*split_username)
        raise LookupError(msg)
    else:
        return password.encode('ascii')