Running PyQGIS script directly (without using interface)?

Question

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.

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

Answer 1

You need to provide some imports and paths that are automatic when you use Python from within QGIS, but not outside. In addition to the libraries you import, start with:

from qgis.core import *

# supply path to where is your qgis installed
QgsApplication.setPrefixPath("/path/to/qgis/installation", True)

# load providers
QgsApplication.initQgis()

see: http://docs.qgis.org/testing/en/docs/pyqgis_developer_cookbook/intro.html, especially "Using PyQGIS in custom application," and "Running custom applications."

Answer 2

You need to create an app object prior to referencing QgsApplication:

from qgis.core import QgsApplication
from PyQt4.QtGui import QApplication
app = QApplication([])