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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 :term:`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 :doc:`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:

    .. literalinclude:: hello_world/app.py
     :linenos:
    
    
  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 :term:`configurator` in a :term:`context manager` to connect :term:`view` code to a particular URL :term:`route`.
  3. Lines 6-8. Implement the view code that generates the :term:`response`.
  4. Lines 15-17. Publish a :term:`WSGI` app using an HTTP server.

As shown in this example, the :term:`configurator` plays a central role in Pyramid development. Building an application from loosely-coupled parts via :ref:`configuration_narr` 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?