Slow selection in QTreeView, why?

Question

I've recently hit a wall in a project I'm working on which uses PyQt. I have a QTreeView hooked up to a QAbstractItemModel which typically has thousands of nodes in it. So far, it works alright, but I realized today that selecting a lot of nodes is very slow. After some digging, it turns out that QAbstractItemModel.parent() is called way too often. I created minimal code to reproduce the problem:

#!/usr/bin/env python
import sys
import cProfile
import pstats

from PyQt4.QtCore import Qt, QAbstractItemModel, QVariant, QModelIndex
from PyQt4.QtGui import QApplication, QTreeView

# 200 root nodes with 10 subnodes each

class TreeNode(object):
    def __init__(self, parent, row, text):
        self.parent = parent
        self.row = row
        self.text = text
        if parent is None: # root node, create subnodes
            self.children = [TreeNode(self, i, unicode(i)) for i in range(10)]
        else:
            self.children = []

class TreeModel(QAbstractItemModel):
    def __init__(self):
        QAbstractItemModel.__init__(self)
        self.nodes = [TreeNode(None, i, unicode(i)) for i in range(200)]

    def index(self, row, column, parent):
        if not self.nodes:
            return QModelIndex()
        if not parent.isValid():
            return self.createIndex(row, column, self.nodes[row])
        node = parent.internalPointer()
        return self.createIndex(row, column, node.children[row])

    def parent(self, index):
        if not index.isValid():
            return QModelIndex()
        node = index.internalPointer()
        if node.parent is None:
            return QModelIndex()
        else:
            return self.createIndex(node.parent.row, 0, node.parent)

    def columnCount(self, parent):
        return 1

    def rowCount(self, parent):
        if not parent.isValid():
            return len(self.nodes)
        node = parent.internalPointer()
        return len(node.children)

    def data(self, index, role):
        if not index.isValid():
            return QVariant()
        node = index.internalPointer()
        if role == Qt.DisplayRole:
            return QVariant(node.text)
        return QVariant()


app = QApplication(sys.argv)
treemodel = TreeModel()
treeview = QTreeView()
treeview.setSelectionMode(QTreeView.ExtendedSelection)
treeview.setSelectionBehavior(QTreeView.SelectRows)
treeview.setModel(treemodel)
treeview.expandAll()
treeview.show()
cProfile.run('app.exec_()', 'profdata')
p = pstats.Stats('profdata')
p.sort_stats('time').print_stats()

To reproduce the problem, just run the code (which does profiling) and select all nodes in the tree widget (either through shift selection or Cmd-A). When you quit the app, the profiling stats will show something like:

Fri May  8 20:04:26 2009    profdata

         628377 function calls in 6.210 CPU seconds

   Ordered by: internal time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    4.788    4.788    6.210    6.210 {built-in method exec_}
   136585    0.861    0.000    1.182    0.000 /Users/hsoft/Desktop/slow_selection.py:34(parent)
   142123    0.217    0.000    0.217    0.000 {built-in method createIndex}
    17519    0.148    0.000    0.164    0.000 /Users/hsoft/Desktop/slow_selection.py:52(data)
   162198    0.094    0.000    0.094    0.000 {built-in method isValid}
     8000    0.055    0.000    0.076    0.000 /Users/hsoft/Desktop/slow_selection.py:26(index)
   161357    0.047    0.000    0.047    0.000 {built-in method internalPointer}
       94    0.000    0.000    0.000    0.000 /Users/hsoft/Desktop/slow_selection.py:46(rowCount)
      404    0.000    0.000    0.000    0.000 /Users/hsoft/Desktop/slow_selection.py:43(columnCount)
       94    0.000    0.000    0.000    0.000 {len}
        1    0.000    0.000    6.210    6.210 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}

The weird part in this data is how often parent() is called: 136k times for 2k nodes! Anyone has a clue why?

Answer 1

Try calling setUniformRowHeights(true) for your tree view:

https://doc.qt.io/qt-4.8/qtreeview.html#uniformRowHeights-prop

Also, there's a C++ tool called modeltest from qt labs. I'm not sure if there is something for python though:

https://wiki.qt.io/Model_Test