I have an application that was originally written in Python 2.7 and used the Python Image Libary (PIL). Now it uses Python 3.x and Pillow (the PIL replacement). One of the things it does is automatically generate .jpg file thumbnails of uploaded files.

It does this by resizing the uploaded image, and then saving it with the filename extension changed.

I started getting an error when uploading a .png file:

OSError: cannot write mode RGBA as JPEG

A bit of research told me that the behavior has changed. Now you need to discard the alpha (transparency) channel of an image before you’re allowed to save it as an image type that doesn’t have an alpha channel. Since PNG files normally have a transparency layer, it makes sense why I would hit that error. It’s trying to protect me from losing data I might potentially want to keep. However, I don’t want or need alpha channel data for my thumbnails.

The solution is just to convert the image before saving.

thumbnail = im.convert('RGB')
thumbnail.save("my_image_filename.jpg", 'JPEG', quality=92)

It’s a simple fix, but slightly annoying that the behavior has changed over the years.

I recently updated a few Django apps from Python 2.7 to Python 3.6 and while going from Django 1.8 to Django 1.11. This is mainly for my own reference, but others may find it useful.

Move the existing virtualenv (this will break the app):

mv ~/.virtualenvs/wbstats ~/.virtualenvs/wbstats2

Create a new virtualenv:

mkvirtualenv -p /usr/bin/python3 wbstats

At this point it might be a good idea to update the version of Django in requirements.txt. For WbStats it moved from 1.8 to 1.11.

Pip install the requirements.txt:

pip install -f requirements.txt

Edit settings.py and make sure there is a TEMPLATES section, which replaces TEMPLATE_DIRS. In addition, TEMPLATE_CONTEXT_PROCESSORS no longer exists. Basically anything with TEMPLATE_ has been rolled into the TEMPLATES section.

You may need to change some imports, i.e.

from models import *

may become:

from stats.models import *

The URLs will probably choke because strings are no longer allowed. Do not import “patterns”. Urlpatterns now is an array: urlpatterns = [url(…), url(…)]

'stats.views.index'

may need to be changed to:

from stats import views
views.index

After imports are fixed, the next thing will probably be fixing print statements.

Urlparse has been moved.

import urlparse

becomes:

import urllib.parse as urlparse

If you updated Django you’ll probably need to run migrations.

python manage.py migrate

Admin commands will need to be ported:

     option_list = BaseCommand.option_list + (make_option('-d', '--domain', default=None, action='store', type='string', dest='domain', help='The name of a domain to create or update.'),)

Becomes:

     def add_arguments(self, parser):
          parser.add_argument('-d', '--domain', default=None, action='store', dest='domain', help='The name of a domain to create or update.')

The parser arguments are mostly the same, but  references to “type=’int'” should be changed “type=int” without quotes, otherwise you’ll get ValueError: ‘int’ is not callable.

There’s some code in some of the admin commands I’ve written that changes the codec for stdout:

UTF8Writer = codecs.getwriter(‘utf8’)
sys.stdout = UTF8Writer(sys.stdout)

This will NEED to be removed because it causes cryptic TypeError problems saying things must be str and not bytes.

render_to_response needs to change to render because context_instance doesn’t exist.