Python PyQt signals are not always working

Question

Hopefully I am following the guidelines correctly here with my first question. I am trying to create a GUI with the MVC structure. I am having difficulty with understanding why my signals are not always being picked up by the controller. I know that there is just something simple that I'm missing. I'm attaching code from a simple calculator which I used as a guide. I removed most of the features to simplify this as much as possible. It is now only 3 of the original buttons and my own button. For debugging, I just have the value on the button printed out when you press it.

import sys

# Import QApplication and the required widgets from PyQt5.QtWidgets
from PySide2.QtWidgets import QApplication
from PySide2.QtWidgets import QMainWindow
from PySide2.QtWidgets import QWidget
from PySide2.QtCore import Qt
from PySide2.QtWidgets import QGridLayout
from PySide2.QtWidgets import QLineEdit
from PySide2.QtWidgets import QPushButton
from PySide2.QtWidgets import QVBoxLayout
from functools import partial

ERROR_MSG = 'ERROR'

# Create a subclass of QMainWindow to setup the calculator's GUI
class PyCalcUi(QMainWindow):
    """PyCalc's View (GUI)."""
    def __init__(self):
        """View initializer."""
        super().__init__()
        # Set some main window's properties
        self.setWindowTitle('PyCalc')
        self.setFixedSize(235, 235)
        # Set the central widget and the general layout
        self.generalLayout = QVBoxLayout()
        self._centralWidget = QWidget(self)
        self.setCentralWidget(self._centralWidget)
        self._centralWidget.setLayout(self.generalLayout)
        # Create the display and the buttons
        self._createDisplay()
        self._createButtons()

    def _createDisplay(self):
        """Create the display."""
        # Create the display widget
        self.display = QLineEdit()
        # Set some display's properties
        self.display.setFixedHeight(35)
        self.display.setAlignment(Qt.AlignRight)
        self.display.setReadOnly(True)
        # Add the display to the general layout
        self.generalLayout.addWidget(self.display)
        
    def _createButtons(self):
        """Create the buttons."""
        self.buttons = {}
        buttonsLayout = QGridLayout()
        # Button text | position on the QGridLayout
        buttons = {'7': (0, 0),
                   '8': (0, 1),
                   '9': (0, 2),
                  }
        # Create the buttons and add them to the grid layout
        for btnText, pos in buttons.items():
            self.buttons[btnText] = QPushButton(btnText)
            self.buttons[btnText].setFixedSize(40, 40)
            buttonsLayout.addWidget(self.buttons[btnText], pos[0], pos[1])
        self.mybutton = QPushButton("5")
        buttonsLayout.addWidget(self.mybutton,1,0)
        # Add buttonsLayout to the general layout
        self.generalLayout.addLayout(buttonsLayout)       
   
# Create a Controller class to connect the GUI and the model
class PyCalcCtrl:
    """PyCalc Controller class."""
    def __init__(self, model, view):
        """Controller initializer."""
        self._evaluate = model
        self._view = view
        # Connect signals and slots
        self._connectSignals()
        
    def _printthis(self):
        print("Hi")
        
    def _printthat(self, buttonvalue):
        print(buttonvalue)
        
    def _connectSignals(self):
        """Connect signals and slots."""
        self._view.mybutton.clicked.connect(self._printthis)
        for btnText, btn in self._view.buttons.items():
            btn.clicked.connect(partial(self._printthat, btnText))


# Create a Model to handle the calculator's operation
def evaluateExpression(expression):
    """Evaluate an expression."""
    try:
        result = str(eval(expression, {}, {}))
    except Exception:
        result = ERROR_MSG

    return result

        
# Client code
def main():
    """Main function."""
    # Create an instance of QApplication if it doesn't exist
    pycalc = QApplication.instance()
    if pycalc is None: 
        pycalc = QApplication(sys.argv)
    # Show the calculator's GUI
    view = PyCalcUi()
    view.show()
    # Create instances of the model and the controller
    model = evaluateExpression
    PyCalcCtrl(model=model, view=view)
    # Execute the calculator's main loop
    sys.exit(pycalc.exec_())

if __name__ == '__main__':
    main()

This set of code works, BUT if I comment out the

        for btnText, btn in self._view.buttons.items():
            btn.clicked.connect(partial(self._printthat, btnText))

The self._view.mybutton.clicked.connect(self._printthis) will no longer work.

What is the btn.clicked.connect(partial(self._printthat, btnText)) line doing which is allowing any other signal I put in def _connectSignals(self): to work. What aspect of that line is achieving something that the mybutton signal isn't doing?

Answer 1

The problem is caused because the PyCalcCtrl object is not assigned to a variable so it will be destroyed and therefore the "_printthis" method will not be accessible. On the other hand, when functools.partial is used then the object of the PyCalcCtrl class is assigned to the scope of that function, that's why it works.

The solution is to assign the PyCalcCtrl object to a variable:

ctrl = PyCalcCtrl(model=model, view=view)