Execution of the process
========================
Each process has to define :meth:`pywps.Process.WPSProcess.execute` method,
which makes the calculation and sets the output values. This method is
called, when Execute request comes to PyWPS.

In principle, you can do what ever you need within this method. It is
advised, to use python bindings to popular GIS packages, like `GRASS GIS
<http://grass.osgeo.org>`_, `GDAL/OGR <http://gdal.org>`_, `PROJ4
<http://proj.remotesensing.org>`_, or any other 
`Python-GIS package <http://pypi.python.org/pypi?:action=browse&show=all&c=391>`_.

In the :ref:`special` chapter, we should give you quick intro to some of
this packages. Some examples are also distributed along with PyWPS source.

If you need to run some shell programs, you should use
:meth:`pywps.Process.WPSProcess.cmd` method for such purpose.

Example of simple execute method. Method will transform raster file from
one coordinate system to another, using Python bindings to GDAL.::

    from osgeo import gdal
    from osgeo.gdalconst import *

    ...

    def execute(self):
        """Convert input raster file to PNG using GDAL"""
      
        # create gdal input dataset 
        indataset = gdal.Open( self.inputRaster.getValue())
        
        # get output driver for PNG format
        pngDriver = gdal.GetDriverByName("png")
        
        # define output gdal dataset
        outfile = "output.png"
        outdataset = pngDriver.CreateCopy(outfile, indataset)

        self.outputRaster.setValue(outfile)
        self.outputRaster.format = {'mimeType':"image/png"}
        
        return