[FIXED] Cannot connect PySide QTableView selectionChanged signal

Issue

I am designing a program composed of a 3D viewer and a table with Python 3.4 and PySide bindings.

I have created a TableView with this class:

from PySide import QtGui
from PySide.QtCore import Qt

class MyTableView(QtGui.QWidget):

    def __init__(self, parent=None):
        super(MyTableView, self).__init__()
        self.parent = parent
        self.title = "Results"
        self.initUI()

    def initUI(self):
        self.grid = QtGui.QGridLayout(self)

        self.table = QtGui.QTableView()
        self.grid.addWidget(self.table, 0, 0)
        # Configure table
        self.table.verticalHeader().setVisible(False)
        self.table.horizontalHeader().setDefaultAlignment(Qt.AlignLeft)
        self.table.setSortingEnabled(True)
        self.table.setAlternatingRowColors(True)
        self.table.setShowGrid(False)
        self.table.setEditTriggers(QtGui.QAbstractItemView.NoEditTriggers)
        self.table.setSelectionBehavior(QtGui.QAbstractItemView.SelectRows)

and the model with this other class:

class MyModel(QStandardItemModel):

    def __init__(self, path, *args, **kwargs):
        super(MyModel, self).__init__()
        self.path = path
        self.parse()

    def parse(self):
        with open(self.path) as f:
            self.mydata = yaml.load(f)

        self.setColumnCount(len(self.mydata['headers']) + 1)
        self.setHorizontalHeaderLabels(
            ['ID'] + self.mydata['headers'])
        row = 0
        for ind, val in self.mydata['rows'].items():
            col = 0
            self.insertRow(row)
            self.setItem(row, col, QStandardItem(ind))
            for v in val:
                col += 1
                self.setItem(row, col, QStandardItem(str(v)))

            row += 1

which are then tied together in this controller:

from PySide.QtCore import Qt

class MyController(object):

    def __init__(self, model, view):
        self.model = model
        self.tableview = view.table
        self.fill_table()
        self.connect_signals()

    def fill_table(self):
        self.tableview.setModel(self.model)
        self.tableview.sortByColumn(0, Qt.AscendingOrder)

    def connect_signals(self):
        selectionModel = self.tableview.selectionModel()
        selectionModel.selectionChanged.connect(self.selection_changed)

    def selection_changed(self, selected, deselected):
        print("Selection changed.")

Then, the program is executed through this script:

def main():
    app = QtGui.QApplication(sys.argv)
    MyController(MyModel(sys.argv[1]), MyView())
    sys.exit(app.exec_())

if __name__ == '__main__':
    main()

(Note that I haven't posted the main window class, but you get the idea)

The table gets rendered OK, but I am not able to connect the selectionChanged signal to the handler (which should udpate the viewer, but for testing purposes it's only a print statement).

What am I doing wrong? Thanks!

[EDIT] I have discovered that it works if I use a lambda function to call the handler method. Can someone explain why?!

selectionModel.selectionChanged.connect(lambda: self.selection_changed(selectionModel.selectedRows()))

Solution

I tried to implement what you wrote and it worked - so I can't be 100% sure of why you are having trouble. But I suspect it is because of what I had to fix to get it to work at all: I had to sort out some problems you have with garbage collection.

In the example code you give, you create a MyController, a MyModel and a MyView. But they will all then be garbage collected (in CPython) since you don't keep a reference to them. If you add a reference to MyController

my_controller = MyController(MyModel(sys.argv[1]), MyView())

you are almost there, but I think the MyTableView might also then be garbage collected since the controller only keeps a reference to the QTableVIew not the MyTableView.

Presumably using the lanbda function changes the references you are preserving - it preserves the controller and the selection model - and that may be why it is working in that case.

Generally it's a good idea to use the Qt parenting mechanism. If you simply parented all these objects on the main window (or their natural parent widget) that would have prevented most of these problems.

Answered By - strubbly