'Sign into Salesforce.
If we have already signed it, this will just return the original object'

def sign_in(self):

    if hasattr(self, 'sf'):
        return self.sf
    sf = Salesforce(username=self.connection.login, password=self.connection.password, security_token=self.extras['security_token'], instance_url=self.connection.host)
    self.sf = sf
    return sf
