Django create thumbnail form ImageField and save in a different ImageField - now with updating!

imagewiththumbnails_updateable.py

# Extension of http://www.yilmazhuseyin.com/blog/dev/create-thumbnails-imagefield-django/

# Note: image_folder and thumbnail_folder are both a callable (ie. a lambda that does a '/'.join())

class Image(Media):
    image = models.ImageField(
        upload_to=image_folder
    )

    thumbnail = models.ImageField(
        upload_to=thumbnail_folder,
        max_length=500,
        null=True,
        blank=True
    )

    def create_thumbnail(self):
        # original code for this method came from
        # http://snipt.net/danfreak/generate-thumbnails-in-django-with-pil/

        # If there is no image associated with this.
        # do not create thumbnail
        if not self.image:
            return

        from PIL import Image
        from cStringIO import StringIO
        from django.core.files.uploadedfile import SimpleUploadedFile
        import os

        # Set our max thumbnail size in a tuple (max width, max height)
        THUMBNAIL_SIZE = (99, 66)

        DJANGO_TYPE = self.image.file.content_type

        if DJANGO_TYPE == 'image/jpeg':
            PIL_TYPE = 'jpeg'
            FILE_EXTENSION = 'jpg'
        elif DJANGO_TYPE == 'image/png':
            PIL_TYPE = 'png'
            FILE_EXTENSION = 'png'

        # Open original photo which we want to thumbnail using PIL's Image
        image = Image.open(StringIO(self.image.read()))

        # We use our PIL Image object to create the thumbnail, which already
        # has a thumbnail() convenience method that contrains proportions.
        # Additionally, we use Image.ANTIALIAS to make the image look better.
        # Without antialiasing the image pattern artifacts may result.
        image.thumbnail(THUMBNAIL_SIZE, Image.ANTIALIAS)

        # Save the thumbnail
        temp_handle = StringIO()
        image.save(temp_handle, PIL_TYPE)
        temp_handle.seek(0)

        # Save image to a SimpleUploadedFile which can be saved into
        # ImageField
        suf = SimpleUploadedFile(os.path.split(self.image.name)[-1],
                temp_handle.read(), content_type=DJANGO_TYPE)
        # Save SimpleUploadedFile into image field
        self.thumbnail.save(
            '%s_thumbnail.%s' % (os.path.splitext(suf.name)[0], FILE_EXTENSION),
            suf,
            save=False
        )

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

        self.create_thumbnail()

        force_update = False

        # If the instance already has been saved, it has an id and we set 
        # force_update to True
        if self.id:
            force_update = True

        # Force an UPDATE SQL query if we're editing the image to avoid integrity exception
        super(Image, self).save(force_update=force_update)

self.image.read() in line 44, sets the pointer to the last byte and causes a problem with Google storage multipart upload, so you can set seek(0).
google storage error: ValueError: Size 266518 was specified but the file-like object only had 0 bytes remaining.

add this at line 45
self.image_file.seek(0)

If anyone has a better solution really like to hear...

You can use this method for creating thumbnail image:

from PIL import Image
from django.db.models.fields.files import ImageFieldFile
from django.core.files.uploadedfile import InMemoryUploadedFile

def _create_thumbnail(image_field: ImageFieldFile, thumbnail_image_field: ImageFieldFile, size: tuple):
    image = Image.open(image_field.file.file)
    image.thumbnail(size=size)
    image_file = BytesIO()
    image.save(image_file, image.format)
    thumbnail_image_field.save(
        image_field.name,
        InMemoryUploadedFile(
            image_file,
            None, '',
            image_field.file.content_type,
            image.size,
            image_field.file.charset,
        ),
        save=False
    )

Django extracts the content-type from the extension of the given filename, not the content_type parameter of SimpleUploadedFile. So, you might as well use the normal ContentFile.

I ended up using the following:

  def resize(self, geom):
    from PIL import Image

    img = Image.open(self.image.path)
    img.thumbnail(geom)

    def image_to_byte_array(img):
      byte_arr = io.BytesIO()
      img.save(byte_arr, format=img.format)
      byte_arr = byte_arr.getvalue()
      return byte_arr
    
    img_byte = image_to_byte_array(img)
    img_file = ContentFile(img_byte)

    new_res = Resource()
    old_filename = os.path.split(self.image.name)[-1]
    new_res.image.save(old_filename,img_file,save=False)

    return new_res

In 2024 this seems to work fine:

    def create_thumbnail(self):
        if not self.image:
            return

        image = Image.open(self.image)
        image.thumbnail((800, 800))
        image_file = BytesIO()
        image.save(image_file, image.format)
        self.thumb.save(
            self.image.name,
            InMemoryUploadedFile(
                image_file,
                None,
                None,
                self.image.file.content_type,
                image_file.tell(),
                self.image.file.charset,
            ),
            save=False,
        )

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