Mocking ftplib.FTP for unit testing Python code

Question

I don\'t know why I\'m just not getting this, but I want to use mock in Python to test that my functions are calling functions in ftplib.FTP correctly. I\'ve simplified everythi

Answer 1

When you do patch(ftplib.FTP) you are patching FTP constructor. dowload_file() use it to build ftp object so your ftp object on which you call login() and cmd() will be mock_ftp.return_value instead of mock_ftp.

Your test code should be follow:

class TestDownloader(unittest.TestCase):

    @patch('ftplib.FTP', autospec=True)
    def test_download_file(self, mock_ftp_constructor):
        mock_ftp = mock_ftp_constructor.return_value
        download_file('ftp.server.local', 'pub/files', 'wanted_file.txt')
        mock_ftp_constructor.assert_called_with('ftp.server.local')
        self.assertTrue(mock_ftp.login.called)
        mock_ftp.cwd.assert_called_with('pub/files')

I added all checks and autospec=True just because is a good practice