================================
01: Single-File Web Applications
================================

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

Background
==========

Microframeworks are all the rage these days. "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:`python_setup`.

#. Create a directory for this step:

   .. code-block:: bash

    (env27)$ 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

    (env27)$ python app.py

#. Open ``http://localhost:6543/`` in your browser.

Analysis
========

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

#. *Line 11*. The ``if __name__ == '__main__':`` is Python's way of
   saying "Start here when running from the command line".

#. *Lines 12-14*. Use Pyramid's :term:`pyramid:configurator` to connect
   :term:`pyramid:view` code to a particular URL
   :term:`pyramid:route`.

#. *Lines 6-7*. Implement the view code that generates the
   :term:`pyramid:response`.

#. *Lines 15-17*. Publish a :term:`pyramid:WSGI` app using an HTTP
   server.

As shown in this example, the :term:`pyramid:configurator` plays a
central role in Pyramid development. Building an application from
loosely-coupled parts via :ref:`pyramid:configuration_narr` is a
central idea in Pyramid, one that we will revisit regularly in this
*Quick Tour*.

Extra Credit
============

#. Why do we do this:

   .. code-block:: python

      print ('Starting up server on http://localhost:6547')

   ...instead of:

   .. code-block:: python

      print 'Starting up server on http://localhost:6547'

#. 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 ``cntrl-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?