.. _qtut_hello_world:
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================================
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01: Single-File Web Applications
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================================
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What's the simplest way to get started in Pyramid? A single-file module. No
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Python packages, no ``pip install -e .``, no other machinery.
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Background
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==========
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Microframeworks were all the rage, until the next shiny thing came along. "Microframework" is a marketing
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term, not a technical one. They have a low mental overhead: they do so little,
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the only things you have to worry about are *your things*.
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Pyramid is special because it can act as a single-file module microframework.
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You can have a single Python file that can be executed directly by Python. But
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Pyramid also provides facilities to scale to the largest of applications.
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Python has a standard called :term:`WSGI` that defines how Python web
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applications plug into standard servers, getting passed incoming requests, and
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returning responses. Most modern Python web frameworks obey an "MVC"
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(model-view-controller) application pattern, where the data in the model has a
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view that mediates interaction with outside systems.
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In this step we'll see a brief glimpse of WSGI servers, WSGI applications,
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requests, responses, and views.
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Objectives
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==========
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- Get a running Pyramid web application, as simply as possible.
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- Use that as a well-understood base for adding each unit of complexity.
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- Initial exposure to WSGI apps, requests, views, and responses.
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Steps
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=====
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#. Make sure you have followed the steps in :doc:`requirements`.
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#. Starting from your workspace directory
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(``~/projects/quick_tutorial``), create a directory for this step:
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.. code-block:: bash
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$ cd ~/projects/quick_tutorial; mkdir hello_world; cd hello_world
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#. Copy the following into ``hello_world/app.py``:
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.. literalinclude:: hello_world/app.py
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:linenos:
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#. Run the application:
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.. code-block:: bash
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$ $VENV/bin/python app.py
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#. Open http://localhost:6543/ in your browser.
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Analysis
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========
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New to Python web programming? If so, some lines in the module merit
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explanation:
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#. *Line 11*. The ``if __name__ == '__main__':`` is Python's way of saying,
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"Start here when running from the command line", rather than when this
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module is imported.
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#. *Lines 12-14*. Use Pyramid's :term:`configurator` in a :term:`context manager` to connect :term:`view`
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code to a particular URL :term:`route`.
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#. *Lines 6-8*. Implement the view code that generates the :term:`response`.
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#. *Lines 15-17*. Publish a :term:`WSGI` app using an HTTP server.
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As shown in this example, the :term:`configurator` plays a central role in
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Pyramid development. Building an application from loosely-coupled parts via
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:ref:`configuration_narr` is a central idea in Pyramid, one that we will
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revisit regularly in this *Quick Tutorial*.
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Extra credit
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============
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#. Why do we do this:
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.. code-block:: python
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print('Incoming request')
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...instead of:
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.. code-block:: python
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print 'Incoming request'
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#. What happens if you return a string of HTML? A sequence of integers?
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#. Put something invalid, such as ``print xyz``, in the view function. Kill
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your ``python app.py`` with ``ctrl-C`` and restart, then reload your
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browser. See the exception in the console?
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#. The ``GI`` in ``WSGI`` stands for "Gateway Interface". What web standard is
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this modelled after?
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