Return old value to combobox with dynamic search and autocompletion - python-3.x

Question

I have a reimplemented comboBox that performs dynamic search and autocompletion (code isn't mine). The problem is when I type something, that doesn't match any value in combobox list and press enter - I receive an empty string. But I wish to receive instead an old value, that was in combobox before I started to type other value. Could anybody help me with that?
Also I want to ask the meaning of 2 strings in ExtendedComboBox class (as long as code isn't mine):
inside function on_completer_activated there is expression if text: ; I can't understand what does it mean, because I always write the whole expression (like if text == True: or something like that)
I don't understand the meaning of [str] in line self.activated[str].emit(self.itemText(index)). I have never seen this kind of construction in pyqt when something in square brackets comes directly after a signal.
code:
from PyQt5 import QtCore, QtGui, QtWidgets
import sys
class ExtendedComboBox(QtWidgets.QComboBox):
def __init__(self, parent=None):
super(ExtendedComboBox, self).__init__(parent)
self.setFocusPolicy(QtCore.Qt.StrongFocus)
self.setEditable(True)
# add a filter model to filter matching items
self.pFilterModel = QtCore.QSortFilterProxyModel(self)
self.pFilterModel.setFilterCaseSensitivity(QtCore.Qt.CaseInsensitive)
self.pFilterModel.setSourceModel(self.model())
# add a completer, which uses the filter model
self.completer = QtWidgets.QCompleter(self.pFilterModel, self)
# always show all (filtered) completions
self.completer.setCompletionMode(QtWidgets.QCompleter.UnfilteredPopupCompletion)
self.setCompleter(self.completer)
# connect signals
self.lineEdit().textEdited.connect(self.pFilterModel.setFilterFixedString)
self.completer.activated.connect(self.on_completer_activated)
# on selection of an item from the completer, select the corresponding item from combobox
def on_completer_activated(self, text):
if text:
index = self.findText(text)
self.setCurrentIndex(index)
self.activated[str].emit(self.itemText(index))
# on model change, update the models of the filter and completer as well
def setModel(self, model):
super(ExtendedComboBox, self).setModel(model)
self.pFilterModel.setSourceModel(model)
self.completer.setModel(self.pFilterModel)
# on model column change, update the model column of the filter and completer as well
def setModelColumn(self, column):
self.completer.setCompletionColumn(column)
self.pFilterModel.setFilterKeyColumn(column)
super(ExtendedComboBox, self).setModelColumn(column)
class ComboBox_Model(QtCore.QAbstractListModel):
def __init__(self, data_list = [], parent = None):
super(ComboBox_Model, self).__init__()
self.data_list = data_list
def rowCount(self, parent):
return len(self.data_list)
def data(self, index, role):
if role == QtCore.Qt.DisplayRole:
row = index.row()
value = self.data_list[row]
return value
if role == QtCore.Qt.EditRole:
row = index.row()
value = self.data_list[row]
return value
class Mainwindow(QtWidgets.QWidget):
def __init__(self):
super().__init__()
self.combobox = ExtendedComboBox()
self.layout_1 = QtWidgets.QHBoxLayout()
self.layout_1.addWidget(self.combobox)
self.setLayout(self.layout_1)
data = ['some text to display', 'other text to display', 'different text']
self.model = ComboBox_Model(data)
self.combobox.setModel(self.model)
if __name__ == '__main__':
app = QtWidgets.QApplication([])
application = Mainwindow()
application.show()
sys.exit(app.exec())

Answer 1

When a combobox is set as editable, by default allows insertion of non existing items at the bottom of the current model when pressing return. Since the model used in that code is not editable, when pressing return with unrecognized text the combobox is unable to add the new item (and select it), which results in setting the index to -1.
You can connect to the embedded QLineEdit returnPressed signal and check whether the current index is valid or not; this is possible because the signal is also previously connected to the combobox insertion, so when you receive the signal the combo has already tried to add the new item and eventually set the (possibly) invalid index.
In order to store the previous index, just connect to the currentIndexChanged() and save it as long as it's greater or equal to 0.
class ExtendedComboBox(QtWidgets.QComboBox):
def __init__(self, parent=None):
# ...
self.lineEdit().returnPressed.connect(self.returnPressed)
self.currentIndexChanged.connect(self.storePreviousIndex)
self.previousIndex = self.currentIndex()
def storePreviousIndex(self, index):
if index >= 0:
self.previousIndex = index
def returnPressed(self):
if self.currentIndex() < 0 or self.currentText() != self.itemText(self.currentIndex()):
self.setCurrentIndex(self.previousIndex)
Note that the second comparison in returnPressed is to add compatibility to the default internal model, in case setModel() is not called and the insertion policy is NoInsert.
About the two final questions:
the if statement checks if the condition is true or not, or, if you want, the condition is not false, as in "not nothing" (aka, False, 0, None); you can do some experiments with simple statements to better understand: if True:, if 1:, if 'something': will all result as valid conditions, while if False:, if 0: or if '': not.
some signals have multiple signatures for their arguments, meaning that the same signal can be emitted more than once, each time with different types of arguments; for example the activated signal of QComboBox is emitted twice, the first time as int with the new current index, then with the new current text; whenever you want to connect to (or emit) an overload that is not the default one, you need to specify the signature in brackets. In the case above, the signal is explicitly emitted for the str signature only (I don't know why the int was not, though). Note that overloaded signals are being gradually removed in Qt (in fact, the [str] signature of activated() is considered obsolete since Qt 5.14).