.. _qtut_hello_world: ================================ 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 ===== #. Make sure you have followed the steps in :doc:`requirements`. #. Starting from your workspace directory (``~/projects/quick_tutorial``), create a directory for this step: .. code-block:: bash cd ~/projects/quick_tutorial; mkdir hello_world; cd hello_world #. Copy the following into ``hello_world/app.py``: .. literalinclude:: hello_world/app.py :linenos: #. Run the application: .. code-block:: bash $VENV/bin/python app.py #. Open http://localhost:6543/ in your browser. Analysis ======== New to Python web programming? If so, some lines in the module merit explanation: #. *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. #. *Lines 12-14*. Use Pyramid's :term:`configurator` in a :term:`context manager` to connect :term:`view` code to a particular URL :term:`route`. #. *Lines 6-8*. Implement the view code that generates the :term:`response`. #. *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 ============ #. Why do we do this: .. code-block:: python print('Incoming request') ...instead of: .. code-block:: python print 'Incoming request' #. What happens if you return a string of HTML? A sequence of integers? #. 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? #. The ``GI`` in ``WSGI`` stands for "Gateway Interface". What web standard is this modelled after?