Web frameworks

At its most fundamental level, a web application consists of a set of programs (or functions) that are executed when the corresponding URL is visited. The output of the program is returned to the visitor and rendered by the browser.

The purpose of web frameworks is to allow developers to build new apps quickly, easily and without mistakes. This is done by providing APIs and tools that reduce and simplify the amount of coding that is required.

The two classic approaches for developing web applications are:

  • Generating HTML[html-w] [html-o] programmatically.
  • Embedding code into HTML pages.

The first model is the one that was followed, for example, by early CGI scripts. The second model is followed, for example, by PHP[php] (where the code is in PHP, a C-like language), ASP (where the code is in Visual Basic), and JSP (where the code is in Java).

Here is an example of a PHP program that, when executed, retrieves data from a database and returns an HTML page showing the selected records:

  1. <html><body><h1>Records</h1><?
  2.   mysql_connect(localhost,username,password);
  3.   @mysql_select_db(database) or die( "Unable to select database");
  4.   $query="SELECT * FROM contacts";
  5.   $result=mysql_query($query);
  6.   mysql_close();
  7.   $i=0;
  8.   while ($i < mysql_numrows($result)) {
  9.     $name=mysql_result($result,$i,"name");
  10.     $phone=mysql_result($result,$i,"phone");
  11.     echo "<b>$name</b><br>Phone:$phone<br /><br /><hr /><br />";
  12.     $i++;
  13.   }
  14. ?></body></html>

The problem with this approach is that code is embedded into HTML, but the very same code also needs to generate additional HTML and to generate SQL statements to query the database, entangling multiple layers of the application and making it difficult to read and maintain. The situation is even worse for Ajax applications, and the complexity grows with the number of pages (files) that make up the application.

The functionality of the above example can be expressed in web2py with two lines of Python code:

  1. def index():
  2.     return HTML(BODY(H1('Records'), db().select(db.contacts.ALL)))

In this simple example, the HTML page structure is represented programmatically by the HTML, BODY, and H1 objects; the database db is queried by the select command; finally, everything is serialized into HTML. Notice that db is not a keyword but a user defined variable. We will use this name consistently to refer to a database connection to avoid confusion.

Web frameworks are typically categorized as one of two types: A “glued” framework is built by assembling (gluing together) several third-party components. A “full-stack” framework is built by creating components designed specifically to be tightly integrated and work together.

web2py is a full-stack framework. Almost all of its components are built from scratch and are designed to work together, but they function just as well outside of the complete web2py framework. For example, the Database Abstraction Layer (DAL) or the template language can be used independently of the web2py framework by importing gluon.dal or gluon.template into your own Python applications. gluon is the name of the web2py module that contains system libraries. Some web2py libraries, such as building and processing forms from database tables, have dependencies on other portions of web2py. web2py can also work with third-party Python libraries, including other template languages and DALs, but they will not be as tightly integrated as the original components.