Django: Request/Response Cycle

A web application or a website revolves around the request-response cycle and Django applications are no exception to this. But it is not just a two step process. Our Django applications needs to go through various stages to return the end user some result. To understand the Django framework better we must understand how the requests are initiated and the end result is served to the end user. In the following sections I am going to explain various stages of requests and the software or code used there.

When setting up a new Django project, one of the first things you’ll do is wire up your URLconfs and set up some views. But what’s actually happening under the hood here? How does Django route traffic to the view, and what part do middlewares play in this cycle?

WSGI

As we know a Web server is a program that uses HTTP (Hypertext Transfer Protocol) to serve the files that form Web pages to users, in response to their requests, which are forwarded by their computers’ HTTPclients.

WSGI is a tool created to solve a basic problem: connecting a web server to a web framework. WSGI has two sides: the ‘server’ side and the ‘application’ side. To handle a WSGI response, the server executes the application and provides a callback function to the application side. The application processes the request and returns the response to the server using the provided callback. Essentially, the WSGI handler acts as the gatekeeper between your web server (Apache, NGINX, etc) and your Django project.

Between the server and the application lie the middlewares. You can think of middlewares as a series of bidirectional filters: they can alter (or short-circuit) the data flowing back and forth between the network and your Django application.

The Big Picture — Data Flow

When the user makes a request of your application, a WSGI handler is instantiated, which:

1. imports your settings.py file and Django’s exception classes.
2. loads all the middleware classes it finds in the MIDDLEWARE_CLASSES or MIDDLEWARES(depending on Django version) tuple located in settings.py
3. builds four lists of methods which handle processing of request, view, response, and exception.
4. loops through the request methods, running them in order
5. resolves the requested URL
6. loops through each of the view processing methods
7. calls the view function (usually rendering a template)
8. processes any exception methods
9. loops through each of the response methods, (from the inside out, reverse order from request middlewares)
10. finally builds a return value and calls the callback function to the web server

Let’s get started.

Layers of Django Application

1. Request Middlewares
2. URL Router(URL Dispatcher)
3. Views
4. Context Processors
5. Template Renderers
6. Response Middlewares

Whenever the request comes in it is handled by the Request middlewares. We can have multiple middlewares. we can find it in project settings(settings.py). Django request middlewares follows the order while processing the request. Suppose if we have request middlewares in the order A, B, C then the request first processed by the middleware A and then B and then C. Django comes up with bunch of default middlewares. We can also write our own or custom middlewares. After request processed by the middlewares it will be submitted to the URL Router or URL dispatcher.

URL Router will take the request from the request middleware and it takes the URL Path from the request. Based on the url path URL router will tries to match the request path with the available URL patterns. These URL patterns are in the form of regular expressions. After matching the URL path with available URL patterns the request will be submitted to the View which is associated with the URL.

Now, we are in business logic layer Views. Views processes the business logic using request and request data(data sent in GET, POST, etc). After processing the request in the view the request is sent context processors, by using the request context processors adds the context data that will help Template Renderers to render the template to generate the HTTP response.

Again the request will send back to the Response middlewares to process it. Response middlewares will process the request and adds or modifies the header information/body information before sending it back to the client(Browser). After the browser will process and display it to the end user.

Middlewares

Middlewares are employed in a number of key pieces of functionality in a Django project: for example :~ we use CSRF middlewares to prevent cross-site request forgery attacks. They’re used to handle session data. Authentication and authorization is accomplished with the use of middlewares. We can write our own middleware classes to shape (or short-circuit) the flow of data through your application.

process_request

Django middlewares must have at least one of the following methods: process_request, process_response, process_view, and process_exception. These are the methods which will be collected by the WSGI Handler and then called in the order they are listed. Let’s take a quick look at django.contrib.auth.middleware.AuthenticationMiddleware, one of the middlewares which are installed by default when you run django-admin.py startproject:

def get_user(request):
 if not hasattr(request, '_cached_user'):
    request._cached_user = auth.get_user(request)
return request._cached_user

class AuthenticationMiddleware(MiddlewareMixin):
	def process_request(self, request):
    assert hasattr(request, 'session'), (

"The Django authentication middleware requires session middleware"

"to be installed. Edit your MIDDLEWARE%s setting to insert "

"‘django.contrib.sessions.middleware.SessionMiddleware’ before "

“‘django.contrib.auth.middleware.AuthenticationMiddleware’.”
	 ) % ("_CLASSES" if settings.MIDDLEWARE is None else "")
    request.user = SimpleLazyObject(lambda: get_user(request))

As you can see, this middleware only works on the ‘request’ step of the data flow to and from your Django application. This middleware first verifies that the session middleware is in use and has been called already, then it sets the userby calling the get_user helper function. As the WSGI Handler iterates through the list of process_requestmethods, it’s building up this requestobject which will eventually be passed into the view, and you’ll be able to reference request.user. Some of the middlewares in settings.py won’t have process_requestmethods. No big deal; those just get skipped during this stage.

process_requestshould either return None (as in this example), or alternately it can return an HttpResponseobject. In the former case, the WSGI Handler will continue processing the process_request methods, the latter will “short-circuit” the process and begin the process_response cycle.

Resolve the URL

Now that the process_request methods have each been called, we now have a request object which will be passed to the view. Before that can happen, Django must resolve the URL and determine which view function to call. This is simply done by regular expression matching. Your settings.pywill have a key called ROOT_URLCONF which indicates the ‘root’ urls.py file, from which you’ll include the urls.py files for each of your apps. URL routing is covered pretty extensively in the Django tutorials so there’s no need to go into it here.

A view has three requirements:

1. It must be callable. It can be a function-based view, or a class-based view which inherits from View the as_view() method to make it callable depending on the HTTP verb (GET, POST, etc)
2. It must accept an HttpRequest object as the first positional argument. This is the result of calling all the process_request and process_viewmiddleware methods.
3. It must return an HttpResponse object, or raise an exception. It’s this response object which is used to kick off the WSGI Handler’s process_view cycle.

process_view

Now that the WSGI Handler knows which view function to call, it once again loops through its list of middleware methods. The process_view method for any Django middleware is declared like this:

process_view(request, view_function, view_args, view_kwargs)

Just like with process_request, the process_view function must return either None or an HttpResponse object (or raise an exception), allowing the WSGI Handler to either continue processing views, or “short-circuiting” and returning a response. Take a look at the source code for CSRF Middleware to see an example of process_view in action. If a CSRF cookie is present, the process_view method returns None and the execution of the view occurs. If not, the request is rejected and the process is short-circuited, resulting in a failure message.

process_exception

If the view function raises an exception, the Handler will loop through its list of process_exception methods. These methods are executed in reverse order, from the last middleware listed in settings.py to the first. If an exception is raised, the process will short-circuit and no other process_exceptionmiddlewares will be called. Usually we rely on the exception handlers provided by Django’s BaseHandler, but you can certainly implement your own exception handling when you write your own custom middleware class.

process_response

At this point, we’ll have an HttpResponse object, either returned by the view or by the list of process_view methods built by the WSGI handler, and it’s time to loop through the response middlewares in turn. This is the last chance any middleware has to modify the data, and it’s executed from the inner layer outward (think of an onion, with the view at the center). Take a look at the cache middleware source code for an example of process_response in action: depending on different conditions in your app (i.e. whether caching is turned off, if we’re dealing with a stream, etc), we’ll want the response stored in the cache or not.

Note: One difference between pre-1.10 Django and later versions: in the older style MIDDLEWARE_CLASSES, every middleware will always have its process_response method called, even if an earlier middleware short-circuited the process. In the new MIDDLEWARES style, only that middleware and the ones which executed before it will have their process_response methods called. Consult the documentation for more details on the differences between MIDDLEWARES and MIDDLEWARE_CLASSES.

All Done!

Finally, Django’s WSGI Handler builds a return value from the HttpResponseobject and executes the callback function to send that data to the web server and out to the user.

So, two key takeaways:

1. Now we know how the view function is matched to a URLconf and what actually calls it (the WSGI Handler).
2. There are four key points you can hook into the request/response cycle through your own custom middleware: process_request’, process_response, process_view, and process_exception. Think of an onion: request middlewares are executed from the outside-in, hit the view at the center, and return through response middlewares back to the surface.

Originally published by Sarthak Kumar at https://medium.com

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Welcome to my blog , hey everyone in this article you learn how to customize the Django app and view in the article you will know how to register  and unregister  models from the admin view how to add filtering how to add a custom input field, and a button that triggers an action on all objects and even how to change the look of your app and page using the Django suit package let’s get started.

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How model queries work in Django

How model queries work in Django

Welcome to my blog, hey everyone in this article we are going to be working with queries in Django so for any web app that you build your going to want to write a query so you can retrieve information from your database so in this article I’ll be showing you all the different ways that you can write queries and it should cover about 90% of the cases that you’ll have when you’re writing your code the other 10% depend on your specific use case you may have to get more complicated but for the most part what I cover in this article should be able to help you so let’s start with the model that I have I’ve already created it.

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Describe each parameter in Django querset

we’re making a simple query for the myModel table so we want to pull out all the information in the database so we have this variable which is gonna hold a return value and we have our myModel models so this is simply the myModel model name so whatever you named your model just make sure you specify that and we’re gonna access the objects attribute once we get that object’s attribute we can simply use the all method and this will return all the information in the database so we’re gonna start with all and then we will go into getting single items filtering that data and go to our command prompt.

Here and we’ll actually start making our queries from here to do this let’s just go ahead and run** Python manage.py shell** and I am in my project file so make sure you’re in there when you start and what this does is it gives us an interactive shell to actually start working with our data so this is a lot like the Python shell but because we did manage.py it allows us to do things a Django way and actually query our database now open up the command prompt and let’s go ahead and start making our first queries.

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Main Pros and Cons of Django As A Web Framework for Python Developers

Django depicts itself as “the web system for fussbudgets with cutoff times”. It was intended to help Python engineers take applications from idea to consummation as fast as could be expected under the circumstances.

It permits fast turn of events on the off chance that you need to make a CRUD application with batteries included. With Django, you won’t need to rehash an already solved problem. It just works and lets you center around your business rationale and making something clients can utilize.

Pros of Django

“Batteries included” theory

The standard behind batteries-included methods normal usefulness for building web applications accompanies the system, not as isolated libraries.

Django incorporates much usefulness you can use to deal with normal web advancement undertakings. Here are some significant level functionalities that Django gives you, which else you need to stay together if you somehow happened to utilize a small scale structure:

ORM

Database relocations

Client validation

Administrator board

Structures

Normalized structure

Django as a system proposes the right structure of an undertaking. That structure helps designers in making sense of how and where to execute any new component.

With a generally acknowledged venture structure that is like numerous tasks, it is a lot simpler to discover online good arrangements or approach the network for help. There are numerous energetic Python designers who will assist you with comprehending any issue you may experience.

Django applications

Django applications (or applications for short) permit designers to separate a task into numerous applications. An application is whatever is introduced by putting in settings.INSTALLED_APPS. This makes it simpler for engineers to add usefulness to the web application by coordinating outer Django applications into the venture.

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Secure of course

Django gives great security assurance out of the crate and incorporates avoidance components for basic assaults like SQL Injection (XSS) and Cross-site Request Forgery (CSRF). You can discover more subtleties in the official security diagram control.

REST structure for building APIs

Django REST Framework, commonly condensed “DRF”, is a Python library for building APIs. It has secluded and adaptable engineering that functions admirably for both straightforward and complex web APIs.

DRF gives a lot of verification and authorization strategies out of the case. It is an adaptable, full-included library with measured and adjustable engineering. It accompanies nonexclusive classes for CRUD tasks and an implicit API program for testing API endpoints.

GraphQL structure for building APIs

Huge REST APIs regularly require a lot of solicitations to various endpoints to recover every single required datum. GraphQL it’s a question language that permits us to share related information in a lot simpler design. For a prologue to GraphQL and an outline of its ideas, if it’s not too much trouble allude to the authority GraphQL documentation.

Graphene-Django gives reflections that make it simple to add GraphQL usefulness to your Django venture. Ordinary Django models, structures, validation, consent arrangements, and different functionalities can be reused to manufacture GraphQL blueprint. It additionally gives an implicit API program for testing API endpoints.

Cons of Django

Django ORM

Django ORM, made before SQLAlchemy existed, is currently much sub-par compared to SQLAlchemy. It depends on the Active Record design which is more regrettable than the Unit of Work design embraced by SQLAlchemy. This implies, in Django, models can “spare” themselves and exchanges are off as a matter of course, they are a bit of hindsight. Peruse more in Why I kind of aversion Django.

Django advances course popularity increses day by day:

Django is huge and is viewed as strong bit of programming. This permits the network to create several reusable modules and applications yet has additionally restricted the speed of advancement of the Django. On head of that Django needs to keep up in reverse similarity, so it advances gradually.

Rundown - Should I use Django as a Python designer?

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Elective, light systems, similar to Flask, while offering a retreat from Django huge biological system and designs, in the long haul can require substantially more additional libraries and usefulness, in the end making many experienced Python engineers winding up wishing they’d began with Django.

Django undertaking’s security and network have become enormously over the previous decade since the system’s creation. Official documentation and instructional exercises are probably the best anyplace in programming advancement. With each delivery, Django keeps on including huge new usefulness.

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