Why does the QgsFeatureIterator crash in my stand alone script?

Question

I'm trying to write a standalone script in qgis 2.10 in ubuntu I have written a script which crashes when I run from bash but works fine from inside the qgis desktop environment. The script is:

#!/usr/bin/python2.7
from qgis.core import *
from platform import python_version
print python_version()
# supply path to where is your qgis installed
QgsApplication.setPrefixPath("/usr", True)
# load providers
QgsApplication.initQgis()

input_file = "/path/to/shape/file.shp"
input_layer = QgsVectorLayer(input_file, "input layer", "ogr")
QgsMapLayerRegistry.instance().addMapLayer(input_layer)
print str(input_layer.featureCount())
print input_layer.dataProvider().crs().authid()
for feature in input_layer.getFeatures():
    pass
print "hello"

QgsApplication.exitQgis()

When I run from within the qgis DE I get:

2.7.6
105777
EPSG:2193
hello

When I run from bash I get:

2.7.6
105777
EPSG:2193
Segmentation fault (core dumped)

The only thing I can guess is my interpreter choice in my shebang but none of the python executables I can find make any difference? and given the response to the version function that seems unlikely. For the shebang I also tried

#!/usr/bin/env python

with the same result.

I've seen the docs on standalone scripts here and I figured out the correct(?) path to the installation from here but I'm stumped as to the cause of this crash.

Answer 1

Thanks to promting by Joseph I had a look at the docs for the application class reference. In order to utilise functions from qgis in your script you need to add a qgis application class reference object.

 app = QgsApplication([],False, None)

after the

QgsApplication.initQgis()

line and

app.exit()

At the end.

The docs describe the parameters but I was able to fill them as shown in order to run the script as a python shell script.

Also note that arguments for the script you write can be passed to the script from the shell as described here. I used the answer by wewa.

Also, thanks to a suggestion by Ole in this SE question I can now run *any custom pyQGIS process/script/function in parallel (not just functions available within the shell as described in my answer to the above linked SE question) using the above script plus the answer here as a wrapper.

Perhaps anyone else doing the same or similar may find this useful too. My next step is to pass those parallel processes to a server(farm) and speed my custom scripts up even more.