I wrote the following script, which runs fine from the QGIS python console (it loads two shapefiles, intersects them, and then finds the areas of the new regions):
import qgis.utils
import qgis.core
from qgis.analysis import *
from PyQt4.QtCore import QVariant
import os
print(os.getcwd())
layer1 = iface.addVectorLayer("/Users/casta2k/ShapefilePractice/precincts/USA_precincts.shp", "precincts", "ogr")
layer2 = iface.addVectorLayer("/Users/casta2k/ShapefilePractice/data_EPSG_4326/Bnd_2015_q1_region.shp", "zipcodes", "ogr")
overlayAnalyzer = QgsOverlayAnalyzer()
overlayAnalyzer.intersection(layer1, layer2, "/Users/casta2k/ShapefilePractice/intersect/intersectOutput.shp")
layer3 = iface.addVectorLayer("/Users/casta2k/ShapefilePractice/intersect/intersectOutput.shp", "intersect", "ogr")
layer = iface.activeLayer()
provider = layer.dataProvider()
areas = [ feat.geometry().area()
for feat in layer.getFeatures() ]
field = QgsField("area", QVariant.Double)
provider.addAttributes([field])
layer.updateFields()
idx = layer.fieldNameIndex('area')
for area in areas:
new_values = {idx : float(area)}
provider.changeAttributeValues({areas.index(area):new_values})
I'd like to be able to run the code directly from a python script, without having to open the GUI. I've tried searching the internet, but I've found nothing helpful. Do I have to start by running QgsApplication.initQgis()? I thought I had to, but I get a segmentation fault. Also, I tried changing iface.addVectorLayer() to QgsVectorLayer(), which seems to work fine in the Python console, but it seems like the files aren't loaded when I run the python script directly.
As you can see, I'm very lost here (the documentation that is available is very poor), so I would appreciate any help. Thanks!
Update
I tried to include what @Paulo suggested, but I'm still getting errors. The code now starts as follows:
import sys
import os
sys.path.append("/Applications/QGIS.app/Contents/Resources/python")
from qgis.core import *
import qgis.utils
from qgis.analysis import *
# supply path to where is your qgis installed
QgsApplication.setPrefixPath("/Applications/QGIS.app/Contents/MacOS", True)
# load providers
QgsApplication.initQgis()
But on the last line (QgsApplication.initQgis()) I'm getting the following error:
QCoreApplication::applicationDirPath: Please instantiate the QApplication object first
QCoreApplication::applicationDirPath: Please instantiate the QApplication object first
Segmentation fault: 11
