Python

• 2 Comments

•

Ever had one of these issues with Pycharm 2018 and Docker?

Couldn't refresh skeletons for remote interpreter
The docker-compose process terminated unexpectedly: /usr/local/bin/docker-compose -f docker-compose.yml -f .PyCharm2018.3/system/tmp/docker-compose.override.8.yml run --rm --name skeleton_generator_643129755 python
Regenerate skeletons

or

can't open file '/opt/.pycharm_helpers/pycharm/django_test_manage.py' + "No such file or directory"

Then you should clear all pycharm helpers from your docker containers and images:

docker ps -a | grep -i pycharm | awk '{print $1}' | xargs docker rm
docker images | grep -i pycharm | awk '{print $3}' | xargs docker rmi

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Inspired by a recent commitstrip: http://www.commitstrip.com/en/2017/05/06/bugs-of-the-future/

Like almost everyone else I had to test my Single Page AngularJS Application, which uses Django Rest Framework as a Backend, with Internet Explorer (11, thankfully). While my SPA works fine in Chrome and Firefox, it does not work very well in Internet Explorer (shocking, lmao).

Anyway, the obvious errors, ranging from needing polyfils to some CSS quirks, were fixed quickly.

But at some point I noticed: Why the hell are changes I make via PUT/POST/PATCH not shown when I make a GET request (retrieving all instances of a model) to the same endpoint afterwards? It kept returning the same data again and again. Where the hell did my changes go? Is my database broken? Is Internet Explorer not firing the PUT/POST/PATCH calls properly?

None of that was true. As it turns out, Internet Explorer 11 caches almost all GET requests to my REST API. I submit a change, I reload the page, and the change has disappeared. Caching at its best.

So I googled and stackoverflowed (is that a word?), and I found some people talking about cache headers. And they are god damn right... Django aswell as Django Rest Framework do not set any cache headers in the response (most likely for a good reason, better be explicit than implicit).

So I tried a couple of the provided solutions, and I must say, I was really unhappy with those approaches. They were either very repetitive (like the ``@never_cache`` decorator added to all my viewsets), or required monkey patches and other sorts of things that I do not like to see. Btw, a cache buster within my JavaScript SPA was a no-go for me.

So I thought to myself: How about I make my whole Django Rest Framework Application "uncachable"? And so I did, with a very few lines of code and as a Django Middleware:

from django.utils.cache import add_never_cache_headers

class DisableClientSideCachingMiddleware(object):
    """
    Internet Explorer / Edge tends to cache REST API calls, unless some specific HTTP headers are added by our
    application.

    - no_cache
    - no_store
    - must_revalidate
    """
    def process_response(self, request, response):
        add_never_cache_headers(response)
        return response

Don't forget to also add this middleware to your settings.

MIDDLEWARE_CLASSES = (
    ...
    'yourapp.middlewares.DisableClientSideCachingMiddleware',
)

Please note that this disables caching of ALL requests coming to your Django Application. If you are serving static files from your Django Application (instead of serving them directly from your webserver), this will affect your performance.

This middleware works for me. I later discovered that somebody else has already had a similar idea, too. The obvious disadvantage here is that it disables caching for all parts of my Django Application. This is fine for me, as I am only using Django Rest Framework and I do not want caching on client side to happen at all, but it might not be okay for some other applications.

Nevertheless, I hope this piece of code helps people that have similar problems. Also, please feel free to share your experiences and solutions to such caching problems with Internet Explorer.

• 1 Comment

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Django has an interesting default behaviour for NullBooleanFields, which are used by django_filters BooleanFilter. While the String 'True' evaluates to Python Boolean True, and the String 'False' evaluates to Python Boolean False, this is not happening for the lowercase variants 'true' and 'false'. This is kind of annoying when you are using DJango Rest Filters, where you would have a REST API call like this (e.g., when calling from JavaScript):

GET /tasks/?show_only_my_tasks=true

This does not work as expected, as "show_only_my_tasks=true" evaluates to "None".

The correct usage according to Djangos NullBooleanField would have been this:

GET /tasks/?show_only_my_tasks=True

To overcome this issue, you can use the following code snippet:

class BetterBooleanSelect(NullBooleanSelect):
    """
    Djangos NullBooleanSelect does not evaluate 'true' to True, and not 'false' to False
    This overwritten NullBooleanSelect allows that
    See https://code.djangoproject.com/ticket/22406#comment:3
    """
    def value_from_datadict(self, data, files, name):
        value = data.get(name)
        return {
            '2': True,
            True: True,
            'true': True,  # added, as NullBooleanSelect does not do that
            'True': True,
            '3': False,
            'false': False,  # added, as NullBooleanSelect does not do that
            'False': False,
            False: False,
        }.get(value)


class BetterBooleanField(forms.NullBooleanField):
    """
    Better Boolean Field that also evalutes 'false' to False and 'true' to True
    """
    widget = BetterBooleanSelect

    def clean(self, value):
        return super(BetterBooleanField, self).clean(value)


class BetterBooleanFilter(django_filters.BooleanFilter):
    """
    This boolean filter allows evaluating 'true' and 'false'
    """
    field_class = BetterBooleanField

In your REST Filter you then only need to write this:

class TaskFilter(BaseFilter):
    """ Filter for Tasks """
    class Meta:
        model = Task

    show_only_my_tasks = BetterBooleanFilter()

•

ForeignKeys need to have the on_delete Attribute set (e.g., to models.CASCADE for a cascading delete)

This also affects existing migrations. If you have migrations that you created with Django 1.8, you will run into errors (as they do not have that attribute set in the migration).

Also see this Ticket: https://code.djangoproject.com/ticket/28677

Apps should specify the "app_name" attribute in their urls.py

If you don't do this, you might run into an error when you include that urls.py in another urls.py

Also see this Ticket: https://code.djangoproject.com/ticket/28691

SessionAuthenticationMiddleware is no longer available

If you have SessionAuthenticationMiddleware MIDDLEWARE listed (you most likely do if you are upgrading from an older Django Version), you will have to remove it from your middleware list (or tuple).

user.is_authenticated() and user.is_anonymous() are no longer available as functions

They are now properties and have to be called without the function parantheses!

More Info

 

 

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Ever wondered why a certain python package was installed?

E.g., when you are installing WeasyPrint you will find that it installs a lot of other libraries, such as cffi, cariocffi and html5lib. With pipdeptree you can visualize this 🙂

pip install pipdeptree

pipdeptree

WeasyPrint==0.40
  - cairocffi [required: >=0.5, installed: 0.8.0]
    - cffi [required: >=1.1.0, installed: 1.11.0]
      - pycparser [required: Any, installed: 2.18]
  - CairoSVG [required: >=1.0.20, installed: 2.0.3]
    - cairocffi [required: Any, installed: 0.8.0]
      - cffi [required: >=1.1.0, installed: 1.11.0]
        - pycparser [required: Any, installed: 2.18]
    - cssselect [required: Any, installed: 1.0.1]
    - lxml [required: Any, installed: 3.8.0]
    - pillow [required: Any, installed: 4.2.1]
      - olefile [required: Any, installed: 0.44]
    - tinycss [required: Any, installed: 0.4]
  - cffi [required: >=0.6, installed: 1.11.0]
    - pycparser [required: Any, installed: 2.18]
  - cssselect2 [required: >=0.1, installed: 0.2.0]
    - tinycss2 [required: Any, installed: 0.6.0]
      - webencodings [required: >=0.4, installed: 0.5.1]
  - html5lib [required: >=0.999999999, installed: 0.999999999]
    - setuptools [required: >=18.5, installed: 36.5.0]
    - six [required: Any, installed: 1.11.0]
    - webencodings [required: Any, installed: 0.5.1]
  - Pyphen [required: >=0.8, installed: 0.9.4]
  - tinycss2 [required: >=0.5, installed: 0.6.0]
    - webencodings [required: >=0.4, installed: 0.5.1]