Creating Your First :app:`Pyramid` Application
In this chapter, we will walk through the creation of a tiny :app:`Pyramid` application. After we're finished creating the application, we'll explain in more detail how it works. It assumes you already have :app:`Pyramid` installed. If you do not, head over to the :ref:`installing_chapter` section.
Here's one of the very simplest :app:`Pyramid` applications:
When this code is inserted into a Python script named helloworld.py and executed by a Python interpreter which has the :app:`Pyramid` software installed, an HTTP server is started on TCP port 8080.
$ /path/to/your/virtualenv/bin/python helloworld.py
C:\> \path\to\your\virtualenv\Scripts\python.exe helloworld.py
This command will not return and nothing will be printed to the console. When port 8080 is visited by a browser on the URL /hello/world, the server will simply serve up the text "Hello world!". If your application is running on your local system, using http://localhost:8080/hello/world in a browser will show this result.
Each time you visit a URL served by the application in a browser, a logging line will be emitted to the console displaying the hostname, the date, the request method and path, and some additional information. This output is done by the wsgiref server we've used to serve this application. It logs an "access log" in Apache combined logging format to the console.
Press Ctrl-C (or Ctrl-Break on Windows) to stop the application.
Now that we have a rudimentary understanding of what the application does, let's examine it piece-by-piece.
The above helloworld.py script uses the following set of import statements:
The script imports the :class:`~pyramid.config.Configurator` class from the :mod:`pyramid.config` module. An instance of the :class:`~pyramid.config.Configurator` class is later used to configure your :app:`Pyramid` application.
Like many other Python web frameworks, :app:`Pyramid` uses the :term:`WSGI` protocol to connect an application and a web server together. The :mod:`wsgiref` server is used in this example as a WSGI server for convenience, as it is shipped within the Python standard library.
The script also imports the :class:`pyramid.response.Response` class for later use. An instance of this class will be used to create a web response.
The above script, beneath its set of imports, defines a function named hello_world.
The function accepts a single argument (request) and it returns an instance of the :class:`pyramid.response.Response` class. The single argument to the class' constructor is a string computed from parameters matched from the URL. This value becomes the body of the response.
This function is known as a :term:`view callable`. A view callable accepts a single argument, request. It is expected to return a :term:`response` object. A view callable doesn't need to be a function; it can be represented via another type of object, like a class or an instance, but for our purposes here, a function serves us well.
A view callable is required to return a :term:`response` object because a response object has all the information necessary to formulate an actual HTTP response; this object is then converted to text by the :term:`WSGI` server which called Pyramid and it is sent back to the requesting browser. To return a response, each view callable creates an instance of the :class:`~pyramid.response.Response` class. In the hello_world function, a string is passed as the body to the response.
In the above script, the following code represents the configuration of this simple application. The application is configured using the previously defined imports and function definitions, placed within the confines of an if statement:
Let's break this down piece-by-piece.
The if __name__ == '__main__': line in the code sample above represents a Python idiom: the code inside this if clause is not invoked unless the script containing this code is run directly from the operating system command line. For example, if the file named helloworld.py contains the entire script body, the code within the if statement will only be invoked when python helloworld.py is executed from the command line.
Using the if clause is necessary -- or at least best practice -- because code in a Python .py file may be eventually imported via the Python import statement by another .py file. .py files that are imported by other .py files are referred to as modules. By using the if __name__ == '__main__': idiom, the script above is indicating that it does not want the code within the if statement to execute if this module is imported from another; the code within the if block should only be run during a direct script execution.
The config = Configurator() line above creates an instance of the :class:`~pyramid.config.Configurator` class. The resulting config object represents an API which the script uses to configure this particular :app:`Pyramid` application. Methods called on the Configurator will cause registrations to be made in an :term:`application registry` associated with the application.
The second line, config.add_view(hello_world, route_name='hello'), registers the hello_world function as a :term:`view callable` and makes sure that it will be called when the hello route is matched.
After configuring views and ending configuration, the script creates a WSGI application via the :meth:`pyramid.config.Configurator.make_wsgi_app` method. A call to make_wsgi_app implies that all configuration is finished (meaning all method calls to the configurator which set up views, and various other configuration settings have been performed). The make_wsgi_app method returns a :term:`WSGI` application object that can be used by any WSGI server to present an application to a requestor. :term:`WSGI` is a protocol that allows servers to talk to Python applications. We don't discuss :term:`WSGI` in any depth within this book, however, you can learn more about it by visiting wsgi.org.
The :app:`Pyramid` application object, in particular, is an instance of a class representing a :app:`Pyramid` :term:`router`. It has a reference to the :term:`application registry` which resulted from method calls to the configurator used to configure it. The :term:`router` consults the registry to obey the policy choices made by a single application. These policy choices were informed by method calls to the :term:`Configurator` made earlier; in our case, the only policy choices made were implied by calls to its add_view and add_route methods.
Finally, we actually serve the application to requestors by starting up a WSGI server. We happen to use the :mod:`wsgiref` make_server server maker for this purpose. We pass in as the first argument '0.0.0.0', which means "listen on all TCP interfaces." By default, the HTTP server listens only on the 127.0.0.1 interface, which is problematic if you're running the server on a remote system and you wish to access it with a web browser from a local system. We also specify a TCP port number to listen on, which is 8080, passing it as the second argument. The final argument is the app object (a :term:`router`), which is the the application we wish to serve. Finally, we call the server's serve_forever method, which starts the main loop in which it will wait for requests from the outside world.
When this line is invoked, it causes the server to start listening on TCP port 8080. The server will serve requests forever, or at least until we stop it by killing the process which runs it (usually by pressing Ctrl-C or Ctrl-Break in the terminal we used to start it).
Our hello world application is one of the simplest possible :app:`Pyramid` applications, configured "imperatively". We can see that it's configured imperatively because the full power of Python is available to us as we perform configuration tasks.