Django Notes: read values from settings in a safe way

Posted on Sat 29 August 2015 in Django, HowTo, Programmazione, Python • Tagged with Django, getattr, Python, settings

Working on Django projects I find very often that many developers access the values that are defined in settings in this way

from django.conf import settings

my_value = settings.MY_SETTING

What happens if MY_SETTING has not been defined in settings.py? The code will raise an error and crash, of course. How can we make the code more reliable? We could try/except the code block that tries to read the value and maybe set a value if we get an exception, but this would not be a clean way to do this job.

A cleaner way to do it is to use getattr in this way:

from django.conf import settings

my_value = getattr(settings, 'MY_SETTING', 'my-default-value')

getattr will try to read MY_SETTING value from settings.py, if the value doesn't exist my_value will be assigned with 'my-default-value'


How to write a custom Django Middleware

Posted on Sun 23 August 2015 in Development • Tagged with Django, HowTo, middleware, Python, tutorial

To understand how a Django Middleware works we need to remember that the basic architecture of Django is composed by a request and a response. A middleware is something that stays in the middle. Let's give a look to the next diagram, taken from official Django documentation:

middleware

Important things to know

There are four important things to know about middlewares:

  • You need to write a class that just inherit from object
  • The order where you place your middleware in settings.py is important: middlewares are processed from top to bottom during a request and from bottom to top during a response.
  • You don't need to implement all the available methods of a middleware. For example you can just implement process_request and process_template_response
  • If you implement process_request and you decide to return an HttpResponse, all the other middlewares, views etc... will be ignored and only your response will be returned

Writing a middleware

In my example I wanted to implement a feature that saves the time when a request is made and the time when a request has been processed, then calculates the time delta and exposes this value in the context so that is accessible from our templates. How to implement a similar feature using a middleware? Here is my example:

from datetime import datetime


class BenchmarkMiddleware(object):
    def process_request(self, request):
        request._request_time = datetime.now()

    def process_template_response(self, request, response):
        response_time = request._request_time - datetime.now()
        response.context_data['response_time'] = abs(response_time)
        return response

Please don't care about how I calculated the time. I'm aware that there are better ways to do it, but I just wanted to keep it simple and show how to implement a simple middleware.

If you want to see a complete example of a project that includes and uses this middleware, here you can find the complete source code: https://github.com/andreagrandi/benchmark-middleware-example

References


Using virtualenv to manage multiple Python/Django environments in the same machine

Posted on Thu 25 April 2013 in HowTo, Linux, Programmazione, Python, Ubuntu (EN) • Tagged with Django, Python, virtualenv

Developing Python applications sometimes it's useful to be able to test the code with different environments, for example a particular version of Python or a specific Django version etc... Setting up many different virtual machines would be really too much work and even using a chroot environment is not what you need in some cases. Thanks to virtualenv is it possible to create a self contained Python environment with all the specific libraries you may need. Using virtualenv is very easy:

  • Creating the virtual environment: virtualenvmyenv --no-site-packages
  • Entering the virtual environment: source myenv/bin/activate
  • Deactivating the virtual environment: deactivate

That's it! Once you're inside the virtual environment you will be using the Python libraries inside it. I suggest you to install all the Python libraries you need using pip.