Django test optimization with no-op PIL engine

The Air Mozilla project is a regular Django webapp. It's reasonably big for a more or less one man project. It's ~200K lines of Python and ~100K lines of JavaScript. There are 816 "unit tests" at the time of writing. Most of them are kinda typical Django tests. Like:

def test_some_feature(self):
    thing = MyModel.objects.create(key='value')
    url = reverse('namespace:name', args=(thing.id,))
    response = self.client.get(url)
    ....

Also, the site uses sorl.thumbnail to automatically generate thumbnails from uploaded images. It's a great library.

However, when running tests, you almost never actually care about the image itself. Your eyes will never feast on them. All you care about is that there is an image, that it was resized and that nothing broke. You don't write tests that checks the new image dimensions of a generated thumbnail. If you need tests that go into that kind of detail, it best belongs somewhere else.

So, I thought, why not fake ALL operations that are happening inside sorl.thumbnail to do with resizing and cropping images.

Here's the changeset that does it. Note, that the trick is to override the default THUMBNAIL_ENGINE that sorl.thumbnail loads. It usually defaults to sorl.thumbnail.engines.pil_engine.Engine and I just wrote my own that does no-ops in almost every instance.

I admittedly threw it together quite quickly just to see if it was possible. Turns out, it was.

# Depends on setting something like:
#    THUMBNAIL_ENGINE = 'airmozilla.base.tests.testbase.FastSorlEngine'
# in your settings specifically for running tests.


from sorl.thumbnail.engines.base import EngineBase


class _Image(object):
    def __init__(self):
        self.size = (1000, 1000)
        self.mode = 'RGBA'
        self.data = '\xa0'


class FastSorlEngine(EngineBase):

    def get_image(self, source):
        return _Image()

    def get_image_size(self, image):
        return image.size

    def _colorspace(self, image, colorspace):
        return image

    def _scale(self, image, width, height):
        image.size = (width, height)
        return image

    def _crop(self, image, width, height, x_offset, y_offset):
        image.size = (width, height)
        return image

    def _get_raw_data(self, image, *args, **kwargs):
        return image.data

    def is_valid_image(self, raw_data):
        return bool(raw_data)

So, was it much faster?

It's hard to measure because the time it takes to run the whole test suite depends on other stuff going on on my laptop during the long time it takes to run the tests. So I ran them 8 times with the old code and 8 times with this new hack.

So rougly 11% faster. Not a lot but it adds up when you're doing test-driven development or debugging where you run a suite or a test over and over as you're saving the files/tests you're working on.

Room for improvement

In my case, it just worked with this simple solution. Your site might do fancier things with the thumbnails. Perhaps we can combine forces on this and finalize a working solution into a standalone package.