I'm struggling with a memory issue on Heroku when running a Django application (with gunicorn).

I have the following code that takes a user-uploaded image, removes all EXIF data, and returns the image ready for it to be uploaded to S3. This is used both as a form data cleaner and when reading base64 data into memory.

def sanitise_image(img): # img is InMemoryUploadedFile
    try:
       image = Image.open(img)
    except IOError:
       return None
    # Move all pixel data into a new PIL image
    data = list(image.getdata())
    image_without_exif = Image.new(image.mode, image.size)
    image_without_exif.putdata(data)

    # Create new file with image_without_exif instead of input image.
    thumb_io = StringIO.StringIO()
    image_without_exif.save(thumb_io, format=image.format)
    io_len = thumb_io.len
    thumb_file = InMemoryUploadedFile(thumb_io, None, strip_tags(img.name), img.content_type,
                                  io_len, None)
    # DEL AND CLOSE EVERYTHING
    thumb_file.seek(0)
    img.close()
    del img
    thumb_io.close()
    image_without_exif.close()
    del image_without_exif
    image.close()
    del image
    return thumb_file

I basically take an InMemoryUploadedFile and return a new one with just the pixel data.

del and closes may be redundant, but they represent my attempt to fix the situation where Heroku memory usage keeps growing and is not released every time this function terminates, even remaining overnight:

Running this on localhost with Guppy and following the tutorial, there are no remaining InMemoryUploadedFiles nor StringIOs nor PIL Image left in the heap, leaving me puzzled.

My suspicion is Python does not release the memory back to the OS, as I've read in multiple threads on SO. Has anyone played around with InMemoryUploadedFile and can give me an explanation as to why this memory is not being released?

When I do not perform this sanitisation, the issue does not occur.

Thanks a lot!