human
Solve the following python programming problem given the following code:
@ mock.patch ('nova.crypto.revoke_cert') 
def test_revoke_certs_by_user(self, mock_revoke) : 
    user_id = 'test_user' 
    project_id = 2 
    file_name = 'test_file' 
    def mock_certificate_get_all_by_user(context, user_id) : 
        return [{ 
            'user_id' : user_id, 
            'project_id' : project_id, 
            'file_name' : file_name, 
}] 
self.stub_out ('nova.db.certificate_get_all_by_user', <MASK>) 
    crypto.revoke_certs_by_user (user_id) 
    mock_revoke.assert_called_once_with (project_id, mock.ANY) 
    
    Given the code above, what is a proper replacement for <MASK>?
assistant
#Here is a proper replacement for <MASK>.
def mock_certificate_get_all_by_user(