Segmentation fault in custom QAbstractItemModel - pyqt4

Question

I've written my own QAbstractItemModel to show a tree in TreeView. It shows the top level items, but when you expand a directory, the app closes, the the following message is written to the console: "Segmentation fault" What am I doing wrong that is causing this. Here is a simplifed version of my code:
#!/usr/bin/env python
import sys
from PyQt4 import QtCore, QtGui
class TreeModel(QtCore.QAbstractItemModel):
NAME = 0
FILEID = QtCore.Qt.UserRole + 1
horizontalHeaderLabels = ["File Name",]
inventory = None
def set_tree(self, inventory, root_item):
self.emit(QtCore.SIGNAL("layoutAboutToBeChanged()"))
self.inventory = inventory
self.id2fileid = []
self.fileid2id = {}
self.dir_children_ids = {}
self.parent_ids = []
# Create internal ids for all items in the tree for use in
# ModelIndex's.
root_fileid = root_item.file_id
self.append_fileid(root_fileid, None)
remaining_dirs = [root_fileid,]
while remaining_dirs:
dir_fileid = remaining_dirs.pop(0)
dir_id = self.fileid2id[dir_fileid]
dir_children_ids = []
for child in inventory[dir_fileid].children:
id = self.append_fileid(child.file_id, dir_id)
dir_children_ids.append(id)
if child.children:
remaining_dirs.append(child.file_id)
if len(self.id2fileid) % 100 == 0:
QtCore.QCoreApplication.processEvents()
self.dir_children_ids[dir_id] = dir_children_ids
self.emit(QtCore.SIGNAL("layoutChanged()"))
def append_fileid(self, fileid, parent_id):
ix = len(self.id2fileid)
self.id2fileid.append(fileid)
self.parent_ids.append(parent_id)
self.fileid2id[fileid] = ix
return ix
def columnCount(self, parent):
if parent.isValid():
return 0
return len(self.horizontalHeaderLabels)
def rowCount(self, parent):
if self.inventory is None:
return 0
parent_id = parent.internalId()
if parent_id not in self.dir_children_ids:
return 0
return len(self.dir_children_ids[parent_id])
def _index(self, row, column, parent_id):
item_id = self.dir_children_ids[parent_id][row]
return self.createIndex(row, column, item_id)
def index(self, row, column, parent = QtCore.QModelIndex()):
if self.inventory is None:
return self.createIndex(row, column, 0)
parent_id = parent.internalId()
return self._index(row, column, parent_id)
def sibling(self, row, column, index):
sibling_id = child.internalId()
if sibling_id == 0:
return QtCore.QModelIndex()
parent_id = self.parent_ids[child_id]
return self._index(row, column, parent_id)
def parent(self, child):
child_id = child.internalId()
if child_id == 0:
return QtCore.QModelIndex()
item_id = self.parent_ids[child_id]
if item_id == 0 :
return self.createIndex(0, 0, item_id)
parent_id = self.parent_ids[item_id]
row = self.dir_children_ids[parent_id].index(item_id)
return self.createIndex(row, 0, item_id)
def hasChildren(self, parent):
if self.inventory is None:
return False
parent_id = parent.internalId()
return parent_id in self.dir_children_ids
def data(self, index, role):
if not index.isValid():
return QtCore.QVariant()
fileid = self.id2fileid[index.internalId()]
if role == self.FILEID:
return QtCore.QVariant(fileid)
item = self.inventory[fileid]
column = index.column()
if column == self.NAME:
if role == QtCore.Qt.DisplayRole:
return QtCore.QVariant(item.file_name)
return QtCore.QVariant()
def flags(self, index):
if not index.isValid():
return QtCore.Qt.ItemIsEnabled
return QtCore.Qt.ItemIsEnabled | QtCore.Qt.ItemIsSelectable
def headerData(self, section, orientation, role):
if orientation == QtCore.Qt.Horizontal and role == QtCore.Qt.DisplayRole:
return QtCore.QVariant(self.horizontalHeaderLabels[section])
return QtCore.QVariant()
inventory = {}
class InventoryItem():
def __init__(self, file_id, file_name, children=[]):
self.file_id = file_id
self.file_name = file_name
self.children = children
global inventory
inventory[file_id] = self
root_item = InventoryItem("root-id", "", [
InventoryItem("dir1-id", "dir1", [
InventoryItem("file1-id", "file1")
]),
InventoryItem("file1-id", "file1")
])
app = QtGui.QApplication(sys.argv)
model = TreeModel()
model.set_tree(inventory, root_item)
tree_view = QtGui.QTreeView()
tree_view.setModel(model)
tree_view.show()
app.exec_()
The full version can be found in this branch: https://code.launchpad.net/~garyvdm/qbzr/trees, in the file lib/browse.py

Answer 1

Use the modeltest.py module!
It exercises your model for different scenarios.
You can find it in the /contrib/ directory inside the PyQt source code package.

Answer 2

Something is being garbage collected by Python which Qt still wants to use. These errors are hard to track down. As a first step, I suggest to add this line before or after setModel(model):
tree_view.model = model
This will keep a reference of the model in the Python part of the tree object.
If that doesn't help, you'll have to strip down your code until it stops crashing.

Answer 3

The problem was that I was returning a valid index for the root in parent() when I should be been returning QtCore.QModelIndex().
It would still be nice to be able to debug this kind of thing, which I still don't know how to do.