[Gfoss] Fwd: [New post] User defined expression functions for QGIS

[Paolo Cavallini]

Salve.
una nuova funzione in QGIS, che consente di utilizzare nel labelling e
nel calcolatore non solo le funzioni preconfezionate, ma anche quelle
definite dall'utente. Non proprio utilissima per chi e' alle prime armi,
ma per i power users e' una ganzata.
Se qualcuno ci inventa utilizzi interessanti, per piacere dia notizia in
lista.
Saluti, e buon divertimento.

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Nathan posted: "Ever since I added expression based labels, including
the new expression builder UI, something that I always wanted to add is
the ability to define custom user defined functions in Python (or C++)
and use them in an expression. The expression engine is "
Respond to this post by replying above this line


    New post on *Nathans QGIS and GIS blog*

	

<http://woostuff.wordpress.com/author/madmanwoo/> 	


    User defined expression functions for QGIS
    <http://woostuff.wordpress.com/2012/11/10/user-defined-expression-functions-for-qgis/>

by Nathan <http://woostuff.wordpress.com/author/madmanwoo/>

Ever since I added expression based labels
<http://woostuff.wordpress.com/2011/10/27/expression-based-labeling/>,
including the new expression builder UI, something that I always wanted
to add is the ability to define custom user defined functions in Python
(or C++) and use them in an expression. The expression engine is used
for labels
<http://woostuff.wordpress.com/2011/10/27/expression-based-labeling/>,
rule
<http://woostuff.wordpress.com/2012/01/25/improvements-to-the-qgis-rule-based-rendering/>
based rendering
<http://woostuff.wordpress.com/2011/06/06/one-of-my-favorite-features-of-qgis/>,
layer actions, and atlas composer tags.  Thanks to the all the awesome
work on the expression engine by Martin all this cool stuff is now possible.

Today I pushed a commit
<https://github.com/qgis/Quantum-GIS/commit/a7699e2696efcb471ab84871aae7af406ca2a375> into
master that adds the ability to define a function in Python (or C++),
register it in the expression engine, then use it anywhere expressions
are used.


    The good stuff

Lets take a use case from Ujaval Gandhi
<http://qgistips.spatialthoughts.com/2012/11/tip-count-number-of-vertices-in-layer.html> and
his example of counting vertices for each feature.

First we need to import the new |qgsfunction|
<https://github.com/qgis/Quantum-GIS/blob/a7699e2696efcb471ab84871aae7af406ca2a375/python/utils.py#L375>
decorator function from |qgis.utils|. The |qgsfunction| decorator will
take a normal Python function, wrap it up in the class used to define a
function, and register it in the engine.

So what does an empty function look like:

from qgis.utils import qgsfunction
from qgis.core import QGis

@qgsfunction(0, "Python")
def vertices(values, feature, parent):
	pass

|@qgsfunction(0, "Python")| means we are defining a new vertices
function that takes 0 args and lives in the "python" group in the
expression builder UI. Any custom function must take |(values, feature,
parent)| as python args. *|values|* is a list of QVariants passed into
the function, *|feature|* is the current |QgsFeature|, and *|parent|* is
expression engine node (you use this to raise errors).

Lets stick some more logic in there:

from qgis.utils import qgsfunction
from qgis.core import QGis

@qgsfunction(0, "Python")
def vertices(values, feature, parent):
	"""
		Returns the number of vertices for a features geometry
	"""
	count = None
	geom = feature.geometry()
	if geom is None: return None
	if geom.type() == QGis.Polygon:
		count = 0
		if geom.isMultipart():
		  polygons = geom.asMultiPolygon()
		else:
		  polygons = [ geom.asPolygon() ]
		for polygon in polygons:
		  for ring in polygon:
		    count += len(ring)
	return count

Pretty simple. Get the geometry from the feature, check if it's a
polygon, if it is then count the number of vertices and return that number.

Now that we have that all done we can save it into a file in our
|.qgis/python| folder, lets call it *userfunctions.py* (note you don't
have to save it here, anywhere that QGIS can find it will do.  Anywhere
on PATH)

Lets open QGIS and run import *userfunctions.py:*

<http://woostuff.files.wordpress.com/2012/11/import.png>

Importing functions from userfunctions.py

Now open the label properties for the layer:

<http://woostuff.files.wordpress.com/2012/11/expression.png>

The new function shown in the expression builder

Nice! Notice also that the function doc string is used as the function
help. How cool is that.  You can also see the $ sign in front of the
function, this is because any functions that take no args are considered
special and use the $ sign as a convention, this is all automatic when
the function is registered.

And the result is:

<http://woostuff.files.wordpress.com/2012/11/result.png>

The label using the new function

You can even use it in the rule based rendering:

<http://woostuff.files.wordpress.com/2012/11/rules.png>

Rule rendering using new function

Enjoy!


    Notes

  * You must unregister a function once you are finished with it using
    QgsExpression.unregisterFunction(name). This mainly applies to
    plugins where the user might unload your plugin and the code is no
    longer available. In the above example we could import userfunctions
    and never unregister because we plan on using it for the whole session.
  * You can't override the built-in methods.

*Nathan <http://woostuff.wordpress.com/author/madmanwoo/>* | November
10, 2012 at 4:38 pm | Categories: Open Source
<http://woostuff.wordpress.com/?cat=25> | URL: http://wp.me/pjIwZ-jc

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