Creating Your First Pyramid Application

In this chapter, we will walk through the creation of a tiny Pyramid application. After we're finished creating the application, we'll explain in more detail how it works. It assumes you already have Pyramid installed. If you do not, head over to the Installing Pyramid section.

Hello World

Here's one of the very simplest Pyramid applications:

from wsgiref.simple_server import make_server
from pyramid.config import Configurator
from pyramid.response import Response


def hello_world(request):
    return Response('Hello World!')


if __name__ == '__main__':
    with Configurator() as config:
        config.add_route('hello', '/')
        config.add_view(hello_world, route_name='hello')
        app = config.make_wsgi_app()
    server = make_server('0.0.0.0', 6543, app)
    server.serve_forever()

When this code is inserted into a Python script named helloworld.py and executed by a Python interpreter which has the Pyramid software installed, an HTTP server is started on TCP port 6543.

On Unix:

$VENV/bin/python helloworld.py

On Windows:

%VENV%\Scripts\python helloworld.py

This command will not return and nothing will be printed to the console. When port 6543 is visited by a browser on the URL /, the server will simply serve up the text "Hello world!". If your application is running on your local system, using http://localhost:6543/ 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.

Imports

The above helloworld.py script uses the following set of import statements:

from wsgiref.simple_server import make_server
from pyramid.config import Configurator
from pyramid.response import Response

Like many other Python web frameworks, Pyramid uses the WSGI protocol to connect an application and a web server together. The wsgiref server is used in this example as a WSGI server for convenience, as it is shipped within the Python standard library.

The script imports the Configurator class from the pyramid.config module. An instance of the Configurator class is later used to configure your Pyramid application.

The script also imports the pyramid.response.Response class for later use. An instance of this class will be used to create a web response.

View Callable Declarations

The above script, beneath its set of imports, defines a function named hello_world.

def hello_world(request):
    return Response('Hello World!')

The function accepts a single argument (request) and it returns an instance of the 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 view callable. A view callable accepts a single argument, request. It is expected to return a 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 always called with a request object. A request object is a representation of an HTTP request sent to Pyramid via the active WSGI server.

A view callable is required to return a 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 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 Response class. In the hello_world function, a string is passed as the body to the response.

Application Configuration

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:

if __name__ == '__main__':
    with Configurator() as config:
        config.add_route('hello', '/')
        config.add_view(hello_world, route_name='hello')
        app = config.make_wsgi_app()
    server = make_server('0.0.0.0', 6543, app)
    server.serve_forever()

Let's break this down piece by piece.

Configurator Construction

if __name__ == '__main__':
    with Configurator() as config:

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 with Configurator() as config: line above creates an instance of the Configurator class using a context manager. The resulting config object represents an API which the script uses to configure this particular Pyramid application. Methods called on the Configurator will cause registrations to be made in an application registry associated with the application.

Adding Configuration

        config.add_route('hello', '/')
        config.add_view(hello_world, route_name='hello')

The first line above calls the pyramid.config.Configurator.add_route() method, which registers a route to the root (/) URL path.

The second line registers the hello_world function as a view callable and makes sure that it will be called when the hello route is matched.

WSGI Application Creation

        app = config.make_wsgi_app()

After configuring views and ending configuration, the script creates a WSGI application via the 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 sets up views and various other configuration settings, have been performed). The make_wsgi_app method returns a WSGI application object that can be used by any WSGI server to present an application to a requestor. WSGI is a protocol that allows servers to talk to Python applications. We don't discuss WSGI in any depth within this book, but you can learn more about it by reading its documentation.

The Pyramid application object, in particular, is an instance of a class representing a Pyramid router. It has a reference to the application registry which resulted from method calls to the configurator used to configure it. The router consults the registry to obey the policy choices made by a single application. These policy choices were informed by method calls to the Configurator made earlier; in our case, the only policy choices made were implied by calls to its add_view and add_route methods.

WSGI Application Serving

    server = make_server('0.0.0.0', 6543, app)
    server.serve_forever()

Finally, we actually serve the application to requestors by starting up a WSGI server. We happen to use the 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 6543, passing it as the second argument. The final argument is the app object (a router), which is 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 6543. 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).

Conclusion

Our hello world application is one of the simplest possible 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.

References

For more information about the API of a Configurator object, see Configurator .

For more information about view configuration, see View Configuration.