Resize Image on Save in Django Before Sending to Amazon S3 - No Lambda function required

The Simple Way Using Pillow and a Memory Buffer

I've been leveling-up on my Django and Python skills lately, and I ran into a fairly common situation, that I could’t find a definitive solution for.

The Dilema

I have a users app with a simple Profile model, and I’m using django-storages to manage uploading my images and other static assets to Amazon S3.

If a users uploads a massive 4K image, that is never going to be displayed larger than 256px wide, I don’t want to have to store that. So I want to resize the image on save, before uploading it to AWS.

For storing images locally, I just have to install Pillow and override the save method in my Profile model like so:

from PIL import Image

def save(self, *args, **kwargs):
    super().save(*args, **kwargs)

    img = Image.open(self.image.path)
    if img.height > 512 or img.width > 512:
        output_size = (512, 512)
        img.thumbnail(output_size)
        img.save(self.image.path)

But it become a bit more complicated when using S3 buckets. I get a

NotImplementedError

Exception Value: This backend doesn't support absolute paths.

The Solution

Most of what I found online suggested removing the Pillow resize and writing an AWS Lambda function to handle the resize on upload. I initially tried that approach, but according to the AWS docs you shouldn’t use the same bucket for input and output, meaning I had to create a second S3 bucket just for resized images. I couldn’t figure out how to get that setup working with django-storages.

A second approach I found mentioned using a buffer to save the resized image into, and then saving that to AWS. The examples of this that I found were either incomplete or used old versions of python. Here is what actually worked for me using Python 3.8, Django 3.1.3 and Pillow 8.0.1:

user/models.py

from app.utils import image_resize

class Profile(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    image = models.ImageField(default="profile-default.png", upload_to="profile_pics")

    def __str__(self):
        return f"{self.user.username} Profile"

    def save(self, *args, **kwargs):
        image_resize(self.image, 512, 512)
        super().save(*args, **kwargs)

app/utils.py

from django.core.files import File
from pathlib import Path
from PIL import Image
from io import BytesIO

image_types = {
    "jpg": "JPEG",
    "jpeg": "JPEG",
    "png": "PNG",
    "gif": "GIF",
    "tif": "TIFF",
    "tiff": "TIFF",
}


def image_resize(image, width, height):
    # Open the image using Pillow
    img = Image.open(image)
    # check if either the width or height is greater than the max
    if img.width > width or img.height > height:
        output_size = (width, height)
        # Create a new resized “thumbnail” version of the image with Pillow
        img.thumbnail(output_size)
        # Find the file name of the image
        img_filename = Path(image.file.name).name
        # Spilt the filename on “.” to get the file extension only
        img_suffix = Path(image.file.name).name.split(".")[-1]
        # Use the file extension to determine the file type from the image_types dictionary
        img_format = image_types[img_suffix]
        # Save the resized image into the buffer, noting the correct file type
        buffer = BytesIO()
        img.save(buffer, format=img_format)
        # Wrap the buffer in File object
        file_object = File(buffer)
        # Save the new resized file as usual, which will save to S3 using django-storages
        image.save(img_filename, file_object)

I’m overriding the save method still, and calling a function I’ve placed in utils.py of my main application. The following happens in the image_resize function:

The image_function checks if the image is too wide or tall and, if it is, saves a resized version first to a memory buffer and then to S3. Back in the save method we call super().save() to save the remaining fields. The super().save() needs to be called after the image.save() or both the original and the resized images will get uploaded to S3.

I hope that was helpful to someone. As always, thanks for reading and Merry Christmas and Happy Holidays!