Process structure
=================
In the file containing the process, there must be one class with the name
:class:`Process`, which is instance of :class:`pywps.Process.WPSProcess` class.
Each process must contain at least two methods:
:meth:`pywps.Process.WPSProcess.__init__` and :meth:`pywps.Process.WPSProcess.execute` .

.. _process-initialization:
Process initialization: __init__ method
---------------------------------------
This method is the constructor for the actual process. It has to invoke the :meth:`pywps.Process.WPSProcess.__init__` method of the superior `WPSProcess` class with process configuration options, described in :class:`pywps.Process.WPSProcess` in more detail.


The process can then define several
:class:`pywps.Process.InAndOutputs.Input` and
:class:`pywps.Process.InAndOutputs.Output` instances. Several methods can be used
for this, namely :meth:`pywps.Process.WPSProcess.addLiteralInput`, :meth:`pywps.Process.WPSProcess.addComplexInput`, :meth:`pywps.Process.WPSProcess.addBBoxInput` for inputs and :meth:`pywps.Process.WPSProcess.addLiteralOutput`, :meth:`pywps.Process.WPSProcess.addComplexOutput`, :meth:`pywps.Process.WPSProcess.addBBoxOutput` for outputs.

Process execution: execute method
---------------------------------
The :meth:`pywps.Process.WPSProcess.execute` method, which is originally
empty, is called by PyWPS for process execution.
The actual calculation is to be done here. When the process
returns any text, it is handled as an error message.  When a process is successfully
calculated, this method returns None.

Example of PyWPS process 
------------------------

.. literalinclude:: ../../../examples/returner.py
   :language: python

After adding `"returner"` string to `__all__` array, in the
:file:`__init__.py`  file in the PyWPS Processes directory, we can try
GetCapabilities, DescribeProcess and Execute requests.