[gdal-dev] Intermittent failure with Python mocks

[Jon Morris]

Yes it does help. I’m still getting to grips with patching and see it as a last resort if you can’t set the test up a better way. I’ll skip the failing tests for now and see if I can rewrite them.

Though I guess the question now is why does it actually work most of the time? 😊

Thanks


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Sent: Thursday, April 13, 2023 3:14 PM
To: Jon Morris <Jon.Morris at jbarisk.com>
Cc: gdal dev <gdal-dev at lists.osgeo.org>
Subject: Re: [gdal-dev] Intermittent failure with Python mocks

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Hi Jon,

The number one gotcha with patching is documented here https://docs.python.org/3/library/unittest.mock.html#where-to-patch. In a nutshell, you have to patch your own module. I see you're patching gdal in your tests. Patching gdal from your test doesn't do anything, it's too late.

Hope this helps!

On Thu, Apr 13, 2023, 7:53 AM Jon Morris <Jon.Morris at jbarisk.com<mailto:Jon.Morris at jbarisk.com>> wrote:
I’m getting intermittent fails with our Python unit tests and I wondered if anyone has seen the same issue. I think it’s a race condition where a mock patch is either not applied at all or maybe applied too slowly so the patched function still has its original return value. It might be a pytest issue, but I’ve now seen it with two different GDAL methods so I’m just curious if anyone has any idea what may be going on.

The first example I reported on Stack Overflow (https://stackoverflow.com/q/75185263/3182496) and looks something like this:

        mock_feat_defn = mocker.patch.object(ogr.FeatureDefn, 'GetFieldCount')
        mock_feat_defn.side_effect = RuntimeError("Some other error message")

        with pytest.raises(RuntimeError, match="Some other error message"):
            get_from_layer_schema(test_lyr, get_names=True, get_types=False)

The second example is this:

        with mock.patch('osgeo.gdal.Driver.Delete', side_effect=None):
            with self.assertRaisesRegex(DatasetError, "Could not delete"):
                delete_path(shp_path)

In both cases we’re mocking a GDAL method call to check one of our functions correctly raises an error, but sometimes it works and sometimes it doesn’t. If it’s not a GDAL issue, no problem, I will keep looking elsewhere for a solution.

Jon
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