(Django admin inline) PIL thumbnail generation w/ smart cropping

admin.py

# Requirements:
# - a model with `file`and `thumbnail` fields.
# - the smartcrop module for Python https://github.com/smartcrop/smartcrop.py

@admin.register(MyModel)
class MyModelAdmin(admin.ModelAdmin):
    """Set MyModel Admin."""
    
    formset = MyModelFormset
  
    def generate_media_thumbnail(self, obj):
      """Generate media thumbnail and return sizes."""

      def get_alpha_composited_image(image):
          """Return image with converted transparency layer if alpha detected."""
          if image.mode == "RGBA":
              background = Image.new("RGBA", image.size, (255, 255, 255))
              image = Image.alpha_composite(background, image)
          image = image.convert("RGB")
          return image

      def get_thumbnail(image):
          """Return smart-cropped image as thumbnail."""
          image = get_alpha_composited_image(image.copy())
          best_crop = SmartCrop().crop(image, THUMBNAIL_SIZE[0], THUMBNAIL_SIZE[1])[
              "top_crop"
          ]
          left = best_crop["x"]
          top = best_crop["y"]
          right = best_crop["width"] + best_crop["x"]
          bottom = best_crop["height"] + best_crop["y"]
          return image.crop((left, top, right, bottom)).resize(
              (THUMBNAIL_SIZE[0], THUMBNAIL_SIZE[1])
          )

      image = Image.open(obj.media.file.file)
      thumbnail = get_thumbnail(image)
      image_file = BytesIO()
      thumbnail.save(image_file, image.format)
      obj.thumbnail.save(obj.media.name, File(image_file), save=False)
      return {
          "original": {"width": image.size[0], "height": image.size[1]},
          "thumbnail": {"width": obj.thumbnail.width, "height": obj.thumbnail.height},
      }

  def save_formset(self, request, form, formset, change):
    """Save inline formset."""
    objs = formset.save(commit=False)
    for obj in formset.deleted_objects:
        obj.delete()
    for obj in objs:
        if isinstance(obj, MyModel):
            sizes = self.generate_media_thumbnail(obj)
    formset.save_m2m()

forms.py

THUMBNAIL_SIZE = (200, 200)
ALLOWED_IMAGE_FORMATS = (".jpg", ".jpeg", ".png")

class MyModelFormset(forms.models.BaseInlineFormSet):
    """Define formset."""

    def file_extension_allowed(self, file_path):
        """Allow JPG, JPEG and PNG image extensions only."""
        return file_path.endswith(ALLOWED_IMAGE_FORMATS)

    def image_larger_than_thumbnail(self, image_data):
        """Validate image dimensions."""
        image_size = get_image_dimensions(image_data)
        return image_size > (
            THUMBNAIL_SIZE[0],
            THUMBNAIL_SIZE[1],
        )

    def clean(self, exclude=None):
        """Validate fields."""
        for form in self.forms:
            media_field_data = form.cleaned_data.get("media", None)
            if media_field_data:
                if not self.file_extension_allowed(media_field_data.name):
                    raise ValidationError(
                        f'Sono supportati {", ".join(list(ALLOWED_IMAGE_FORMATS))}'
                    )
                if not self.image_larger_than_thumbnail(media_field_data):
                    raise ValidationError(
                        f"L'immagine inserita non può essere più piccola di "
                        f"{THUMBNAIL_SIZE[0]}x"
                        f"{THUMBNAIL_SIZE[1]} px"
                    )