pyQGIS segfault

Question

I am trying to get started with pyQGIS but failing pretty hard.
Trying to follow http://docs.qgis.org/testing/en/docs/pyqgis_developer_cookbook/intro.html
A lot of it seems incomplete for Windows users, especially if they happen to have another python install...

OS: Windows10
QGIS: 2.10.1
PATH:

    C:\Python27\;
    C:\Python27\Scripts;
    C:\Program Files\QGIS Pisa\bin;
    C:\Program Files\QGIS Pisa\apps\qgis\bin;

PYTHONPATH:

    C:\Program Files\QGIS Pisa\apps\qgis\python;
    C:\Program Files\QGIS Pisa\apps\qgis\python\plugins;
    C:\Program Files\QGIS Pisa\apps\Python27\lib\

test.py:

    from qgis.core import *

    # supply path to where is your qgis installed
    QgsApplication.setPrefixPath("C:/program files/qgis pisa/apps/qgis", True)

    # load providers
    QgsApplication.initQgis()

    data_source = "..." #many different files.
    layer_name = "nctasdfasd"
    provider_name = "ogr"

    layer = QgsVectorLayer(data_source, layer_name, provider_name)

    if not layer.isValid():
      print "Layer failed to load!"

    QgsApplication.exitQgis()

cmdline: /cygdrive/c/Program\ Files/QGIS\ Pisa/bin/python.exe test.py
(Have tried from cygwin and win cmd - no diff)

output:
If i leave the shapefile zipped, get some warnings about recoding CP_OEMCP to UTF-8. If i unzip and point at .shp file, no output. If i run in cygwin, get "Segmentation fault", If i run in cmd, get windows crash dialog.

test shapefiles:
http://gis.nct.opendata.arcgis.com/datasets/438b8aeacb42463f9e3f1ff308ee507b_2
random census data from census.gov cb_2014_us_cd114_500k

Is there any way to get more information about the crash/failure? Getting to this point took a long time fighting silent errors about the python env and qgis prefixpath. I suspect there may still be an issue there, but don't know how to test/debug.

Edit: this is unlikely to help, but I upgraded to Essen and attached VS2015 to get a call stack. Is there a debug release available so I could get symbol names?

QtCore4.dll!00000000720d1e10()  Unknown
qca.dll!00007ff84af58af7()  Unknown
qca.dll!00007ff84af5dec7()  Unknown
qca.dll!00007ff84af5e5c3()  Unknown
qca.dll!00007ff84af41d74()  Unknown
qgis_core.dll!00007ff840a9305f()    Unknown
qgis_core.dll!00007ff840a9841f()    Unknown
_core.pyd!00007ff84160dba9()    Unknown
python27.dll!000000001e0c20a9() Unknown
python27.dll!000000001e112524() Unknown
python27.dll!000000001e115cd4() Unknown
python27.dll!000000001e1174d9() Unknown
python27.dll!000000001e117579() Unknown
python27.dll!000000001e143a9a() Unknown
python27.dll!000000001e144d8a() Unknown
python27.dll!000000001e1453e0() Unknown
python27.dll!000000001e145e23() Unknown
python27.dll!000000001e044c25() Unknown
python.exe!000000001d00119e()   Unknown

More edits: By adding this statement before setPrefix:

app=QgsApplication( [], False)

I got another error message about the GDAL_DATA env which I set to

C:\Program Files\QGIS Essen\share\epsg_csv

However, I still cannot load a layer. Still just crashing.

Answer 1

OK, I fixed my issues. the Bold items were the stuff I was missing.

1. Make sure PATH and PYTHONPATH are set as above. It may be useful to create a new env var 'QGIS'

   C:\Program Files\QGIS Essen
   

2. Create an instance of QgsApplication

   app=QgsApplication( [], False)
   

   use this instance to setPrefix, init, and close:

   app.setPrefixPath("C:/program files/qgis essen/apps/qgis", True)
   app.initQgis()
   
   #Other stuff
   
   app.exitQgis()
   

3. Make sure GDAL_DATA is set to epsg_csv folder:

    C:\Program Files\QGIS Essen\share\epsg_csv
   

Entire working script is here:

import os
from PyQt4.QtCore import *
from PyQt4.QtGui import *
from qgis.core import *


app=QgsApplication( [], False)

# supply path to where is your qgis installed
app.setPrefixPath( os.environ["QGIS"] + "/apps/qgis", True)

# load providers
app.initQgis()

data_source = "" #set path to .shp file!
layer_name = "nct"
provider_name = "ogr"

layer = QgsVectorLayer(data_source, layer_name, provider_name)
if not layer.isValid():
    print "Layer failed to load!"
else:
    print "valid!"
app.exitQgis()