PyQt instance variables in constructor

Question

In "Rapid GUI Programming with Python and Qt", the author gives this example in Chapter 4.

It seems to me that dial, spinbox, and layout would go out of scope. Why can we use dial instead of self.dial, spinbox instead of self.spinbox and layout instead of self.layout? Is this a good practice or is it better to use self. for all GUI elements?

import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *

class Form(QDialog):

    def __init__(self, parent=None):
        super(Form, self).__init__(parent)

        dial = QDial()
        dial.setNotchesVisible(True)
        spinbox = QSpinBox()

        layout = QHBoxLayout()
        layout.addWidget(dial)
        layout.addWidget(spinbox)
        self.setLayout(layout)

        self.connect(dial, SIGNAL("valueChanged(int)"),
                 spinbox.setValue)
        self.connect(spinbox, SIGNAL("valueChanged(int)"),
                 dial.setValue)
        self.setWindowTitle("Signals and Slots")

app = QApplication(sys.argv)
form = Form()
form.show()
app.exec_()

sebastian · Accepted Answer · 2019-01-28 15:33:07Z

1

Yes, those variables go out of scope.

But since there are references to the underlying objects (via layout and self.setLayout) this is not really an issue, i.e. the widgets/objects are not destroyed.

If you don't need to access them directly in other pieces of your code, then it's just fine like that.

answered Jan 28, 2019 at 15:33

sebastian

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2 Comments

nathanesau Over a year ago

So in general, if there are references to underlying objects, those underlying objects will go out of scope when the object referring to them goes out of scope?Is that how this works in Python?

Peter Over a year ago

Yeah, if you delete the layout then the widgets will be removed from memory too.

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