QGIS » QGIS Application

#!/usr/bin/env python

import os

import sys

qgisprefix = '/usr'

# configure paths for QGIS 

os.environ['PATH'] = qgisprefix+'/bin'

os.environ['LD_LIBRARY_PATH'] = qgisprefix+'/lib'

sys.path.insert(0, qgisprefix+'/share/qgis/python')

sys.path.insert(1, qgisprefix+'/share/qgis/python/plugins')

# disable QGIS debug messages

os.environ['QGIS_DEBUG'] = '-1'

# import QGIS modules

from qgis.core import *

from qgis.gui import *

# configure QGIS paths

QgsApplication.setPrefixPath(qgisprefix, True)

# initalise QGIS

QgsApplication.initQgis()

# create a new application object

# without this importing processing causes the following error:

# QPixmap: Must construct a QApplication before a QPaintDevice

app = QgsApplication([], True)

# import PyQt4

from PyQt4 import QtCore, QtGui

# import processing module

import processing

# dummy iface

class DummyInterface(object):

    def __init__(self):

        self.destCrs = None

    def __getattr__(self, *args, **kwargs):

        def dummy(*args, **kwargs):

            return DummyInterface()

        return dummy

    def __iter__(self):

        return self

    def next(self):

        raise StopIteration

    def layers(self):

        # simulate iface.legendInterface().layers()

        return QgsMapLayerRegistry.instance().mapLayers().values()

iface = DummyInterface()

# initalise processing plugin with dummy iface object

plugin = processing.classFactory(iface)

# create a memory layer and add a point to it

layer = QgsVectorLayer("Point?crs=EPSG:27700", "temporary_points", "memory")

feature = QgsFeature()

feature.setGeometry(QgsGeometry.fromPoint(QgsPoint(42, 42)))

layer.dataProvider().addFeatures([feature])

layer.updateExtents()

print 'source:', layer.source()

print 'isValid:', layer.isValid()

# add the layer to the map layer registry

# required for memory layers which can't be accessed by uri alone

QgsMapLayerRegistry.instance().addMapLayer(layer)

# run the 'fixed distance buffer' algorithm, distance=1, segments=100

ret = processing.runalg('qgis:fixeddistancebuffer', layer, 1, 100, False, None)

# load the result

output = processing.load(ret['OUTPUT'])

print 'output:', ret['OUTPUT']

print 'isValid:', output.isValid()

print 'features:', output.dataProvider().featureCount()

# calculate the total area of the features

total_area = 0

for feature in output.getFeatures():

    total_area += feature.geometry().area()

print 'total area (should approximate pi):', total_area

# exit QGIS gracefully

QgsApplication.exitQgis()