How to set a default task from a list of options in a QComboBox?

Question

EDITED

I have two options in my QComboBox which are "feet" and "meters". The app crashes(python stopped working) each time I select the first option on the GUI or if I proceed without selecting any of the two since the value is in "feet" already.

I have the following edited lines of code;

from PyQt5 import QtCore, QtWidgets

class Widget(QtWidgets.QWidget):
def __init__(self):
    super().__init__()

    self.value = QtWidgets.QLineEdit()
    self.unit = QtWidgets.QComboBox()
    self.convert = QtWidgets.QPushButton()

    self.setLayout(QtWidgets.QVBoxLayout())

    self.value.setObjectName("value")
    self.layout().addWidget(self.value)

    self.layout().addWidget(self.unit)
    self.unit.addItems(["feet", "meters"])

    self.layout().addWidget(self.convert)
    self.convert.setText("Convert")

    self.unit.currentIndexChanged.connect(self.unit_select)
    self.convert.clicked.connect(self.convert_click)

# UNIT SELECT
def unit_select(self):
    current_index = self.unit.currentIndex()
    if current_index == 0:
        num = float(self.value.text()) * 0.3047972654
    elif current_index == 1:
        num = float(self.value.text()) * 1
    self.final = num

# CONVERT VALUE
def convert_click(self):
    print("Converted value =", self.final)



if __name__ == '__main__':
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = Widget()
    w.show()
    sys.exit(app.exec_())

I need to set "feet" as the default of the QComboBox.

Answer 1

Try it:

from PyQt5 import QtCore, QtWidgets

class Widget(QtWidgets.QWidget):
    def __init__(self):
        super().__init__()

        self.unit = QtWidgets.QComboBox()

        self.setLayout(QtWidgets.QVBoxLayout())
        self.layout().addWidget(self.unit)
        self.unit.addItems(["feet", "meters"])

        self.unit.setCurrentIndex(0)                            # <-----
        self.current_index = 0                                  # <-----

        self.unit.activated[int].connect(self.onActivatedIndex)
        self.unit.currentIndexChanged.connect(self.unit_select)

        self.unit_select(self.current_index)

    @QtCore.pyqtSlot(int)
    def onActivatedIndex(self, index):
        print("ActivatedIndex", index)

    @QtCore.pyqtSlot(int)
    def unit_select(self, index):

        if index == 0:
            print("\nCurrentIndex {} - feet".format(index))
            # ...
        elif index == 1:
            print("\nCurrentIndex {} - meters".format(index))
            # ...            

if __name__ == '__main__':
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = Widget()
    w.show()
    sys.exit(app.exec_())

---

Update:

import re      # To test string conversion to float  !!!

from PyQt5 import QtCore, QtWidgets

class Widget(QtWidgets.QWidget):
    def __init__(self):
        super().__init__()

        self.value   = QtWidgets.QLineEdit()
        self.unit    = QtWidgets.QComboBox()
        self.convert = QtWidgets.QPushButton()

        self.setLayout(QtWidgets.QVBoxLayout())

        self.value.setObjectName("value")
        self.layout().addWidget(self.value)

        self.layout().addWidget(self.unit)
        self.unit.addItems(["feet", "meters"])

        self.layout().addWidget(self.convert)
        self.convert.setText("Convert")

        self.unit.currentIndexChanged.connect(self.unit_select)
        self.convert.clicked.connect(self.convert_click)

    # UNIT SELECT
    def unit_select(self):

        if re.match("^\d+?\.*\d*?$", self.value.text()) is None: #self.value.text():
            QtWidgets.QMessageBox.information(None, 'Warning', 
                'The string QLineEdit `{}` cannot be converted to float'.format(self.value.text()))
            return False            

        current_index = self.unit.currentIndex()
        if current_index == 0:
            num = float(self.value.text()) * 0.3047972654
        elif current_index == 1:
            num = float(self.value.text()) * 1
        self.final = num

    # CONVERT VALUE
    def convert_click(self):

        if re.match("^\d+?\.*\d*?$", self.value.text()) is None: #self.value.text():
            QtWidgets.QMessageBox.information(None, 'Warning', 
                'The string QLineEdit `{}` cannot be converted to float'.format(self.value.text()))
        else:
            self.unit_select()
            print("Converted value =", self.final)


if __name__ == '__main__':
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = Widget()
    w.show()
    sys.exit(app.exec_())