Python: Dictionary content changes without declaration

Question

I have a Problem in this function:

def show_selected_button(self):
        attrbs = required_attributes[self.modeComboBox.currentText()]
        layers = QgsProject.instance().mapLayersByName(self.layerComboBox.currentText())
        layer = layers[0]
        layer.selectAll()
        selected_features = layer.getSelectedFeatures()
        data = []
        for i in selected_features:
            attribute_row = []
            for a in attrbs:
                attrb = i.attribute(name=a)
                if a == "MITTLEREBR":
                    attrb = round(attrb, 2)
                attribute_row.append(attrb)

            laenge = i.geometry().length()
            vol = laenge * i.attribute(name="DICKE") / 100 * i.attribute(name="MITTLEREBR")
            attribute_row.append(round(laenge, 2))
            attribute_row.append(round(vol, 2))
            data.append(attribute_row)

        attrbs.append("LÄNGE")
        attrbs.append("VOLUMEN")
        self.tableWidget.setRowCount(len(data))
        self.tableWidget.setColumnCount(len(attrbs))
        self.tableWidget.setHorizontalHeaderLabels(attrbs)

        for i, x in enumerate(data):
            for j, y in enumerate(x):
                self.tableWidget.setItem(i, j, QtWidgets.QTableWidgetItem(str(y)))

When I execute this function for the first time (when I click the button), everything works fine but when I use one more time after the first usage, required_attributes["Strasse"] includes the values "LÄNGE" and "VOLUMEN" that originally got added to attrbs and never got added to the dictionary.

Content of required_attributes["Strasse"] when function is run for the first time: ['EBENE', 'SCHICHTTYP', 'SCHICHTT01', 'SCHICHTT02', 'DICKE', 'MITTLEREBR']

Content of required_attributest["Strasse"] when function is run a second time: ['EBENE', 'SCHICHTTYP', 'SCHICHTT01', 'SCHICHTT02', 'DICKE', 'MITTLEREBR', 'LÄNGE', 'VOLUMEN']

required_attributes is a variable stored in a different file.

I don't know why this is happening, can anyone help me?

Ivan Sidaruk · Accepted Answer · 2022-10-26T15:31:41.683

1

By assigning to attrbs value of required_attributes[self.modeComboBox.currentText()] you've created a link to the same object in memory, so whenever you change attrbs now you are changing list inside dictionary.

attrbs = required_attributes[self.modeComboBox.currentText()]

Check this answer for an example: https://stackoverflow.com/a/13531087/10438425

edited Oct 26 '22 at 15:31

answered Oct 26 '22 at 15:25

Ivan Sidaruk

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1

Thank you so much for pointing this out, Ivan! I just added `[:]` to the assignment of attrbs, and now it works :) – Malik Meshi Oct 26 '22 at 15:31

Python: Dictionary content changes without declaration

1 Answers1