Here's my sample code that shows the problem - apologies if this is a big load of code but this is as minimal as I can get it:

import sys
from qgis.core import *
from qgis.gui import *
from PyQt4.QtCore import *
from PyQt4.QtGui import *

app = QgsApplication([], True)
app.setPrefixPath("/usr", True)

class OgrLayer(QgsVectorLayer):
    def __init__(self, path, name, connect_remove):
        QgsVectorLayer.__init__(self, path, name, 'ogr')
        if connect_remove:
    def layerRemoved(self, id):
        print "removed ",id

layer1 = QgsVectorLayer(sys.argv[2],'layer1','ogr')
layer2 = OgrLayer(sys.argv[3],'layer2', sys.argv[1]=="break")

if not layer1.isValid():
    raise ValueError("layer 1 not valid")
if not layer2.isValid():
    raise ValueError("layer 2 not valid")


canvas = QgsMapCanvas()
canvas.setLayerSet([QgsMapCanvasLayer(layer1), QgsMapCanvasLayer(layer2)])


When run as:

python layertest.py ok shape1.shp shape2.shp

it runs, presents an empty canvas window (because I've not bothered to set the extent) and quits cleanly when I close the window.

When run as:

python layertest.py break shape1.shp shape2.shp

it runs as before, but on closing the window I get a segmentation violation. The only difference is the attachment of the layerRemoved signal to the map layer registry within my OgrLayer class.

If I define the OgrLayer class in a Qgis application python console, it all works and Qgis quits cleanly on exit. Its only when running from a standalone python script that this code segfaults on exit.

Is something not initialised properly? Is something in the QApplication exit code trying to access a layer that has been deleted from memory?

The real situation I have is in trying to use a plugin that defines layers in this way with connections - so any solution would ideally not involve "don't connect the signal" since that would mean editing that plugin (and probably breaking it).

I can disconnect the signal after creating the layer and that seems to work, but that's a bit ugly. I'd rather find out what the underlying problem is.

Obviously seg faults on exit aren't as much of a problem as seg faults in the middle of a program, but I'd like to know what the underlying problem is and fix it.

1 Answer 1


It looks like you forgot to match your call to app.initQgis() with a call to app.exitQgis(). Add that after the call to app.exec_() and the segfault disappears.

  • Doh! Maybe we need a context manager to handle the init and exit steps...
    – Spacedman
    Jul 16, 2016 at 7:31

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