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Web application frameworks, or simply "web frameworks", are the de facto way to build web-enabled applications. From simple blogs to complex AJAX-rich applications, every page on the web was created by writing code. I've recently found that many developers interested in learning a web framework like Flask or Django don't really understand what a web framework is, what their purpose is, or how they work. In this article, I'll explore the oft-overlooked topic of web framework fundamentals. By the end of the article, you should have a solid understanding of what a web framework is and why they exist in the first place. This will make it far easier to learn a new web framework and make an informed decision regarding which framework to use.

How The Web Works

Before we talk about frameworks, we need to understand how the web "works". To do so, we'll delve into what happens when you type a URL into your browser and hit Enter. Open a new tab in your browser and navigate to http://www.jeffknupp.com. Let's talk about the steps your browser took in order to display the page (minus DNS lookups).

Web Servers and ... web ... servers...

Every web page is transmitted to your browser as HTML, a language used by browsers to describe the content and structure of a web page. The application responsible for sending HTML to browsers is called a web server. Confusingly, the machine this application resides on is also usually called a web server.

The important thing to realize, however, is that at the end of the day, all a web application really does is send HTML to browsers. No matter how complicated the logic of the application, the final result is always HTML being sent to a browser (I'm purposely glossing over the ability for applications to respond with different types of data, like JSON or CSS files, as the concept is the same).

How does the web application know what to send to the browser? It sends whatever the browser requests.

HTTP

Browsers download websites from web servers (or "application servers") using the HTTP protocol (a protocol, in the realm of programming, is a universally known data format and sequence of steps enabling communication between two parties). The HTTP protocol is based on a request-response model. The client (your browser) requests data from a web application that resides on a physical machine. The web application in turn responds to the request with the data your browser requested.

An important point to remember is that communication is always initiated by the client (your browser). The server (web server, that is) has no way of initiating a connection to you and sending your browser unsolicited data. If you receive data from a web server, it is because your browser explicitly asked for it.

HTTP Methods

Every message in the HTTP protocol has an associated method (or verb). The various HTTP methods correspond to logically different types of requests the client can send, which in turn represent different intentions on the client side. Requesting the HTML of a web page, for example, is logically different than submitting a form, so the two actions require the use of different methods.

HTTP GET

The GET method does exactly what it sounds like: gets (requests) data from the web server. GET requests are the by far the most common HTTP request. During a GET request the web application shouldn't need to do anything more than respond with the requested page's HTML. Specifically, the web application should not alter the state of the application as a result of a GET request (for example, it should not create a new user account based on a GET request). For this reason, GET requests are usually considered "safe" since they don't result in changes to the application powering the website.

HTTP POST

Clearly, there is more to interacting with web sites than simply looking at pages. We are also able to send data to the application, e.g. via a form. To do so, a different type of request is required: POST. POST requests usually carry data entered by the user and result in some action being taken within the web application. Signing up for a web site by entering your information on a form is done by POSTing the data contained in the form to the web application.

Unlike a GET request, POST requests usually result in the state of the application changing. In our example, a new user account is created when the form is POSTed. Unlike GET requests, POST requests do not always result in a new HTML page being sent to the client. Instead, the client uses the response's response code do determine if the operation on the application was successful.

HTTP Response Codes

In the normal case, a web server returns a response code of 200, meaning, "I did what you asked me to and everything went fine". Response codes are always a three digit numerical code. The web applications must send one with each response to indicate what happened as a result of a given request. The response code 200 literally means "OK" and is the code most often used when responding to a GET request. A POST request, however, may result in code 204 ("No Content") being sent back, meaning "Everything went OK but I don't really have anything to show you."

It's important to realize that POST requests are still sent to a specific URL, which may be different from the page the data was submitted from. Continuing our sign up example, the form may reside at www.foo.com/signup. Hitting submit, however, may result in a POST request with the form data being sent to www.foo.com/process_signup. The location a POST request should be sent to is specified in the form's HTML.

Web Applications

You can get quite far using only HTTP GET and POST, as they're the two most common HTTP methods by a wide margin. A web application, then, is responsible for receiving an HTTP request and replying with an HTTP response, usually containing HTML that represents the page requested. POST requests cause the web application to take some action, perhaps adding a new record in the database. There are a number of other HTTP methods, but we'll focus on GET and POST for now.

What would the simplest web application look like? We could write an application that listened for connections on port 80 (the well-known HTTP port that almost all HTTP traffic is sent to). Once it received a connection it would wait for the client to send a request, then it might reply with some very simple HTML.

Here's what that would look like:

import socket

HOST = ''
PORT = 80
listen_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
listen_socket.bind((HOST, PORT))
listen_socket.listen(1)
connection, address = listen_socket.accept()
request = connection.recv(1024)
connection.sendall("""HTTP/1.1 200 OK
Content-type: text/html


<html>
    <body>
        <h1>Hello, World!</h1>
    </body>
</html>""")
connection.close()

(If the above doesn't work, try changing the PORT to something like 8080)

This code accepts a single connection and a single request. Regardless of what URL was requested, it responds with an HTTP 200 response (so it's not really a web server). The Content-type: text/html line represents a header field. Headers are used to supply meta-information about the request or response. In this case, we're telling the client that the data that follows is HTML (rather than, say, JSON).

Anatomy of a Request

If I look at the HTTP request I sent to test the program above, I find it looks quite similar to the response. The first line is <HTTP Method> <URL> <HTTP version> or, in this case, GET / HTTP/1.1. After the first line come a few headers like Accept: */* (meaning we will accept any type of content in a response). That's basically it.

The reply we send has a similar first request line, in the format <HTTP version> <HTTP Status-Code> <Status-Code Reason-Phrase> or HTTP/1.1 200 OK in our case. Next come headers, in the same format as the request headers. Lastly, the actual content of the response is included. Note that this can be encoded as a string or binary object (in the case of files). The Content-type header lets the client know how to interpret the response.

Web Server Fatigue

If we were going to continue building on the example above as the basis for a web application, there are a number of problems we'd need to solve:

1. How do we inspect the requested URL and return the appropriate page?
2. How do we deal with POST requests in addition to simple GET requests
3. How do we handle more advanced concepts like sessions and cookies?
4. How do we scale the application to handle thousands of concurrent connections?

As you can imagine, no one wants to solve these problems each time they build a web application. For that reason, packages exist that handle the nitty-gritty details of the HTTP protocol and have sensible solutions to problems the problems above. Keep in mind, however, at their core they function in much the same way as our example: listening for requests and sending HTTP responses with some HTML back.

Note that client-side web frameworks are a much different beast and deviate significantly from the above description.

Solving The Big Two: Routing and Templates

Of all the issues surrounding building a web application, two stand out.

1. How do we map a requested URL to the code that is meant to handle it?
2. How do we create the requested HTML dynamically, injecting calculated values or information retrieved from a database?

Every web framework solves these issues in some way, and there are many different approaches. Examples will be helpful, so I'll discuss Django and Flask's solutions to both of these problems. First, though, we need to briefly discuss the MVC pattern.

MVC in Django

Django makes use of the MVC pattern and requires code using the framework to do the same. MVC, or "Model-View-Controller" is simply a way of logically separating the different responsibilities of the application. Resources like database tables are represented by models (in much the same way a class in Python often models some real-world object). controllers contain the business logic of the application and operate on models. Views are given all of the information they needs to dynamically generate the HTML representation of the page.

Somewhat confusingly, in Django, controllers are called views and views are called templates. Other than naming weirdness, Django is a pretty straightforward implementation of an MVC architecture.

Routing in Django

Routing is the process of mapping a requested URL to the code responsible for generating the associated HTML. In the simplest case, all requests are handled by the same code (as was the case in our earlier example). Getting a little more complex, every URL could map 1:1 to a view function. For example, we could record somewhere that if the URL www.foo.com/bar is requested, the function handle_bar() is responsible for generating the response. We could build up this mapping table until all of the URLs our application supports are enumerated with their associated functions.

However, this approach falls flat when the URLs contain useful data, such as the ID of a resource (as is the case in www.foo.com/users/3/). How do we map that URL to a view function, and at the same time make use of the fact that we want to display the user with ID 3?

Django's answer is to map URL regular expressions to view functions that can take parameters. So, for example, I may say that URLs that match ^/users/(?P<id>\d+)/$ calls the display_user(id) function where the id argument is the captured group id in the regular expression. In that way, any /users/<some number>/ URL will map to the display_user function. These regular expressions can be arbitrarily complex and include both keyword and positional parameters.

Routing in Flask

Flask takes a somewhat different approach. The canonical method for hooking up a function to a requested URL is through the use of the route() decorator. The following Flask code will function identically to the regex and function listed above:

@app.route('/users/<id:int>/')
def display_user(id):
    # ...

As you can see, the decorator uses an almost simplified form of regular expression to map URLs to arguments (one that implicitly uses / as separators). Arguments are captured by including a <name:type> directive in the URL passed to route(). Routing to static URLs like /info/about_us.html is handled as you would expect: @app.route('/info/about_us.html')

HTML Generation Through Templates

Continuing the example above, once we have the appropriate piece of code mapped to the correct URL, how do we dynamically generate HTML in a way that still allows web designers to hand-craft it? For both Django and Flask, the answer is through HTML templating.

HTML Templating is similar to using str.format(): the desired output is written with placeholders for dynamic values. These are later replaced by arguments to the str.format() function. Imagine writing an entire web page as a single string, marking dynamic data with braces, and calling str.format() at the end. Both Django templates and jinja2, the template engine Flask uses, are designed to be used in this way.

However, not all templating engines are created equal. While Django has rudimentary support for programming in templates, Jinja2 basically lets you execute arbitrary code (it doesn't really, but close enough). Jinja2 also aggressively caches the result of rendering templates, so that subsequent requests with the exact same arguments are returned from the cache instead of expensively being re-rendered.

Database Interaction

Django, with its "batteries included" philosophy, includes an ORM ("Object Relational Mapper"). The purpose of an ORM is two-fold: it maps Python classes to database tables and abstracts away the differences between various database engines (though the former is its primary role). No one loves ORMs (because the mapping between domains is never perfect), rather, they are tolerated. Django's is reasonably full-featured. Flask, being a "micro-framework", does not include one (though it is quite compatible with SQLAlchemy, the Django ORM's biggest/only competitor).

The inclusion of an ORM gives Django the ability to create a full-featured CRUD application. CRUD (Create Read Update Delete) applications seem to be the sweet spot for web frameworks (on the server side). Django (and Flask-SQLAlchemy) make the various CRUD operations for each model straightforward.

Web Framework Round-Up

By now, the purpose of web frameworks should be clear: to hide the boilerplate and infrastructural code related to handling HTTP requests and responses. Just how much is hidden depends on the framework. Django and Flask represent two extremes. Django includes something for every situation, almost to its detriment. Flask bills itself as a "micro-framework" and handles the bare minimum of web application functionality, relying on third-party packages to do some of the less common web framework tasks.

Remember, though, that at the end of the day, Python web frameworks all work the same way: they receive HTTP requests, dispatch code that generates HTML, and creates an HTTP response with that content. In fact, all major server-side frameworks work in this way (excluding JavaScript frameworks). Hopefully, you're now equipped to choose between frameworks as you understand their purpose.