01: Single-File Web Applications

What's the simplest way to get started in Pyramid? A single-file module. No Python packages, no pip install -e ., no other machinery.

Background

Microframeworks were all the rage, until the next shiny thing came along. "Microframework" is a marketing term, not a technical one. They have a low mental overhead: they do so little, the only things you have to worry about are your things.

Pyramid is special because it can act as a single-file module microframework. You can have a single Python file that can be executed directly by Python. But Pyramid also provides facilities to scale to the largest of applications.

Python has a standard called WSGI that defines how Python web applications plug into standard servers, getting passed incoming requests, and returning responses. Most modern Python web frameworks obey an "MVC" (model-view-controller) application pattern, where the data in the model has a view that mediates interaction with outside systems.

In this step we'll see a brief glimpse of WSGI servers, WSGI applications, requests, responses, and views.

Objectives

  • Get a running Pyramid web application, as simply as possible.

  • Use that as a well-understood base for adding each unit of complexity.

  • Initial exposure to WSGI apps, requests, views, and responses.

Steps

  1. Make sure you have followed the steps in Requirements.

  2. Starting from your workspace directory (~/projects/quick_tutorial), create a directory for this step:

    cd ~/projects/quick_tutorial; mkdir hello_world; cd hello_world

  3. Copy the following into hello_world/app.py:

     1from waitress import serve
 2from pyramid.config import Configurator
 3from pyramid.response import Response
 4
 5
 6def hello_world(request):
 7    print('Incoming request')
 8    return Response('<body><h1>Hello World!</h1></body>')
 9
10
11if __name__ == '__main__':
12    with Configurator() as config:
13        config.add_route('hello', '/')
14        config.add_view(hello_world, route_name='hello')
15        app = config.make_wsgi_app()
16    serve(app, host='0.0.0.0', port=6543)

  4. Run the application:

    $VENV/bin/python app.py

  5. Open http://localhost:6543/ in your browser.

Analysis

New to Python web programming? If so, some lines in the module merit explanation:

  1. Line 11. The if __name__ == '__main__': is Python's way of saying, "Start here when running from the command line", rather than when this module is imported.

  2. Lines 12-14. Use Pyramid's configurator in a context manager to connect view code to a particular URL route.

  3. Lines 6-8. Implement the view code that generates the response.

  4. Lines 15-17. Publish a WSGI app using an HTTP server.

As shown in this example, the configurator plays a central role in Pyramid development. Building an application from loosely-coupled parts via Application Configuration is a central idea in Pyramid, one that we will revisit regularly in this Quick Tutorial.

Extra credit

  1. Why do we do this:

    print('Incoming request')

    ...instead of:

    print 'Incoming request'

  2. What happens if you return a string of HTML? A sequence of integers?

  3. Put something invalid, such as print xyz, in the view function. Kill your python app.py with ctrl-C and restart, then reload your browser. See the exception in the console?

  4. The GI in WSGI stands for "Gateway Interface". What web standard is this modelled after?