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.. _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 ``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:`requirements`.
#. Starting from your workspace directory
(``~/projects/quick_tutorial``), create a directory for this step:
.. code-block:: bash
$ 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 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` 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 Tour*.
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 ``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?
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