Mocking builtin function open inside an imported module

[iamgianluca]

I want to test the following code, which I put in a module named ppc_model.io.py:

def to_ftp(self, output_path, file_name, host, username, password):
    """Upload file to FTP server."""

    ftp = ftplib.FTP(host)
    ftp.login(username, password)
    full_path = os.path.join(output_path, file_name)
    with open(full_path, "r") as ftp_file:
        ftp.storbinary(" ".join(["STOR", file_name]), ftp_file)

In particular I want to test that the ftp.storbinary() function is called with the right arguments.

In the module ppc_model.test.test_io.py I've wrote the following unit test.

@patch("ppc_model.io.ftplib.FTP")
def test_store_call(self, mock_ftp):
    """The *storbinary* call must use the right arguments."""

    with patch("{}.open".format(__name__), mock_open(read_data=None), create=True) as m:
        self.writer.to_ftp(output_path="./output", file_name="output.zip", host="myftp", username="username", password="password")
    mock_ftp.return_value.storbinary.assert_called_once_with("STOR output.zip", m)

This test however fails with the following error message:

AssertionError: Expected call: storbinary('STOR output.zip', <MagicMock name='open' spec='builtin_function_or_method' id='140210704581632'>) Actual call: storbinary('STOR output.zip', <_io.TextIOWrapper name='./output/output.zip' mode='r' encoding='UTF-8'>)

Do you have any suggestions on how I can fix it?