.. index::
   single: hello world program

.. _firstapp_chapter:

Creating Your First :app:`Pyramid` Application
==============================================

In this chapter, we will walk through the creation of a tiny :app:`Pyramid`
application.  After we're finished creating the application, we'll explain in
more detail how it works. It assumes you already have :app:`Pyramid` installed.
If you do not, head over to the :ref:`installing_chapter` section.

.. _helloworld_imperative:

Hello World
-----------

Here's one of the very simplest :app:`Pyramid` applications:

.. literalinclude:: helloworld.py
   :linenos:

When this code is inserted into a Python script named ``helloworld.py`` and
executed by a Python interpreter which has the :app:`Pyramid` software
installed, an HTTP server is started on TCP port 6543.

On Unix:

.. code-block:: bash

    $VENV/bin/python helloworld.py

On Windows:

.. code-block:: doscon

    %VENV%\Scripts\python helloworld.py

This command will not return and nothing will be printed to the console. When
port 6543 is visited by a browser on the URL ``/``, the server will
simply serve up the text "Hello world!".  If your application is running on
your local system, using `<http://localhost:6543/>`_ in a browser
will show this result.

Each time you visit a URL served by the application in a browser, a logging
line will be emitted to the console displaying the hostname, the date, the
request method and path, and some additional information.  This output is done
by the wsgiref server we've used to serve this application.  It logs an "access
log" in Apache combined logging format to the console.

Press ``Ctrl-C`` (or ``Ctrl-Break`` on Windows) to stop the application.

Now that we have a rudimentary understanding of what the application does,
let's examine it piece by piece.

Imports
~~~~~~~

The above ``helloworld.py`` script uses the following set of import statements:

.. literalinclude:: helloworld.py
   :lineno-match:
   :lines: 1-3

Like many other Python web frameworks, :app:`Pyramid` uses the :term:`WSGI`
protocol to connect an application and a web server together.  The
:mod:`wsgiref` server is used in this example as a WSGI server for convenience,
as it is shipped within the Python standard library.

The script imports the :class:`~pyramid.config.Configurator` class from the
:mod:`pyramid.config` module.  An instance of the
:class:`~pyramid.config.Configurator` class is later used to configure your
:app:`Pyramid` application.

The script also imports the :class:`pyramid.response.Response` class for later
use.  An instance of this class will be used to create a web response.

View Callable Declarations
~~~~~~~~~~~~~~~~~~~~~~~~~~

The above script, beneath its set of imports, defines a function named
``hello_world``.

.. literalinclude:: helloworld.py
   :lineno-match:
   :pyobject: hello_world

The function accepts a single argument (``request``) and it returns an instance
of the :class:`pyramid.response.Response` class.  The single argument to the
class' constructor is a string computed from parameters matched from the URL. 
This value becomes the body of the response.

This function is known as a :term:`view callable`.  A view callable accepts a
single argument, ``request``.  It is expected to return a :term:`response`
object.  A view callable doesn't need to be a function; it can be represented
via another type of object, like a class or an instance, but for our purposes
here, a function serves us well.

A view callable is always called with a :term:`request` object.  A request
object is a representation of an HTTP request sent to :app:`Pyramid` via the
active :term:`WSGI` server.

A view callable is required to return a :term:`response` object because a
response object has all the information necessary to formulate an actual HTTP
response; this object is then converted to text by the :term:`WSGI` server
which called Pyramid and it is sent back to the requesting browser.  To return
a response, each view callable creates an instance of the
:class:`~pyramid.response.Response` class.  In the ``hello_world`` function, a
string is passed as the body to the response.

.. index::
   single: imperative configuration
   single: Configurator
   single: helloworld (imperative)

.. _helloworld_imperative_appconfig:

Application Configuration
~~~~~~~~~~~~~~~~~~~~~~~~~

In the above script, the following code represents the *configuration* of this
simple application. The application is configured using the previously defined
imports and function definitions, placed within the confines of an ``if``
statement:

.. literalinclude:: helloworld.py
   :lineno-match:
   :lines: 10-16

Let's break this down piece by piece.

Configurator Construction
~~~~~~~~~~~~~~~~~~~~~~~~~

.. literalinclude:: helloworld.py
   :lineno-match:
   :lines: 10-11

The ``if __name__ == '__main__':`` line in the code sample above represents a
Python idiom: the code inside this if clause is not invoked unless the script
containing this code is run directly from the operating system command line.
For example, if the file named ``helloworld.py`` contains the entire script
body, the code within the ``if`` statement will only be invoked when ``python
helloworld.py`` is executed from the command line.

Using the ``if`` clause is necessary—or at least best practice—because code in
a Python ``.py`` file may be eventually imported via the Python ``import``
statement by another ``.py`` file.  ``.py`` files that are imported by other
``.py`` files are referred to as *modules*.  By using the ``if __name__ ==
'__main__':`` idiom, the script above is indicating that it does not want the
code within the ``if`` statement to execute if this module is imported from
another; the code within the ``if`` block should only be run during a direct
script execution.

The ``with Configurator() as config:`` line above creates an instance of the
:class:`~pyramid.config.Configurator` class using a :term:`context manager`.  The resulting ``config`` object
represents an API which the script uses to configure this particular
:app:`Pyramid` application.  Methods called on the Configurator will cause
registrations to be made in an :term:`application registry` associated with the
application.

.. _adding_configuration:

Adding Configuration
~~~~~~~~~~~~~~~~~~~~

.. literalinclude:: helloworld.py
   :lineno-match:
   :lines: 12-13

The first line above calls the :meth:`pyramid.config.Configurator.add_route`
method, which registers a :term:`route` to the root (``/``) URL path.

The second line registers the ``hello_world`` function as a :term:`view
callable` and makes sure that it will be called when the ``hello`` route is
matched.

.. index::
   single: make_wsgi_app
   single: WSGI application

WSGI Application Creation
~~~~~~~~~~~~~~~~~~~~~~~~~

.. literalinclude:: helloworld.py
   :lineno-match:
   :lines: 14

After configuring views and ending configuration, the script creates a WSGI
*application* via the :meth:`pyramid.config.Configurator.make_wsgi_app` method.
A call to ``make_wsgi_app`` implies that all configuration is finished
(meaning all method calls to the configurator, which sets up views and various
other configuration settings, have been performed).  The ``make_wsgi_app``
method returns a :term:`WSGI` application object that can be used by any WSGI
server to present an application to a requestor. :term:`WSGI` is a protocol
that allows servers to talk to Python applications.  We don't discuss
:term:`WSGI` in any depth within this book, but you can learn more about it by
reading its `documentation <https://wsgi.readthedocs.io/en/latest/>`_.

The :app:`Pyramid` application object, in particular, is an instance of a class
representing a :app:`Pyramid` :term:`router`.  It has a reference to the
:term:`application registry` which resulted from method calls to the
configurator used to configure it.  The :term:`router` consults the registry to
obey the policy choices made by a single application.  These policy choices
were informed by method calls to the :term:`Configurator` made earlier; in our
case, the only policy choices made were implied by calls to its ``add_view``
and ``add_route`` methods.

WSGI Application Serving
~~~~~~~~~~~~~~~~~~~~~~~~

.. literalinclude:: helloworld.py
   :lineno-match:
   :lines: 15-16

Finally, we actually serve the application to requestors by starting up a WSGI
server.  We happen to use the :mod:`wsgiref` ``make_server`` server maker for
this purpose.  We pass in as the first argument ``'0.0.0.0'``, which means
"listen on all TCP interfaces".  By default, the HTTP server listens only on
the ``127.0.0.1`` interface, which is problematic if you're running the server
on a remote system and you wish to access it with a web browser from a local
system.  We also specify a TCP port number to listen on, which is 6543, passing
it as the second argument.  The final argument is the ``app`` object (a
:term:`router`), which is the application we wish to serve.  Finally, we call
the server's ``serve_forever`` method, which starts the main loop in which it
will wait for requests from the outside world.

When this line is invoked, it causes the server to start listening on TCP port
6543.  The server will serve requests forever, or at least until we stop it by
killing the process which runs it (usually by pressing ``Ctrl-C`` or
``Ctrl-Break`` in the terminal we used to start it).

Conclusion
~~~~~~~~~~

Our hello world application is one of the simplest possible :app:`Pyramid`
applications, configured "imperatively".  We can see that it's configured
imperatively because the full power of Python is available to us as we perform
configuration tasks.

References
----------

For more information about the API of a :term:`Configurator` object, see
:class:`~pyramid.config.Configurator` .

For more information about :term:`view configuration`, see
:ref:`view_config_chapter`.
