Python3 descriptors don't call __get__ when accessed - python-3.x

Question

Can anyone tell me why when I push test button, __get__ isnt called? I tried many things but I can't achieve desired behavior:
when lt or gt attribute accessed the value of appropriate QlineEdit must be returned if checkbox is checked
#!/usr/bin/python3
# -*- coding: utf-8 -*-
import sys
from PyQt5.QtWidgets import QApplication
from PyQt5.QtWidgets import QVBoxLayout, QRadioButton, QGroupBox, QLineEdit, QCheckBox, QGridLayout, QLabel, QFrame, QPushButton
from PyQt5.QtCore import Qt
from PyQt5.QtGui import QIntValidator
from PyQt5.QtWidgets import QMainWindow
class LineDescriptor:
def __init__(self, row=1, label=None):
self.label = label
self.row = row
self.__name = None
def __get__(self, instance, owner=None):
print('get_called')
if instance is None:
print('is none')
return self
else:
private_name = '_{0}__{1}'.format(type(instance).__name__,
self.__name)
check_name = '_{0}__{1}'.format(type(instance).__name__,
self.__name)
if getattr(instance, check_name).isChecked():
return getattr(instance, private_name).text()
else:
return None
class CheckSelector(QGroupBox):
def __init__(self, name):
self.__dictionary = []
super(CheckSelector, self).__init__()
layout = QGridLayout()
layout.setAlignment(Qt.AlignTop)
name = QLabel(name)
layout.addWidget(name, 0, 0, 1, 1)
self.__check = QCheckBox()
layout.addWidget(self.__check, 0, 1, 1, 1)
self.setLayout(layout)
for name, attribute in type(self).__dict__.items():
if isinstance(attribute, LineDescriptor):
row = attribute.row
if attribute.label is not None:
label = QLabel(attribute.label)
self.layout().addWidget(label, row, 0, 1, 1)
int_validator = QIntValidator()
field = QLineEdit()
field.setValidator(int_validator)
layout.addWidget(field, row, 1, 1, 1)
private_name = '_{0}__{1}'.format(type(self).__name__, name)
setattr(attribute, '_LineDescriptor__name', name)
setattr(self, name, attribute)
setattr(self, private_name, field)
print(attribute)
print(attribute.__dict__)
class GtCheck(CheckSelector):
gt = LineDescriptor(1, '>')
class BothCheck(CheckSelector):
gt = LineDescriptor(1, '>')
lt = LineDescriptor(2, '<')
def checker():
print(test.lt)
app = QApplication(sys.argv)
win = QMainWindow()
wid = QFrame()
lay = QVBoxLayout()
test = BothCheck(name='слов')
lay.addWidget(test)
but = QPushButton()
but.clicked.connect(checker)
lay.addWidget(but)
wid.setLayout(lay)
win.setCentralWidget(wid)
win.show()
sys.exit(app.exec_())

Answer 1

Here's the offending line:
setattr(self, name, attribute)
Your descriptor is a non-data descriptor (it does not implement __set__()). Thus, this sets the attribute in self.__dict__, creating an instance attribute. Instance attributes take precedence over non-data descriptors. If you immediately did this on the next line:
getattr(self, name)
...then the descriptor would not be invoked; it would just fetch attribute back out of self.__dict__ and return it directly. Since foo.bar is exactly the same as getattr(foo, "bar"), the foo.bar syntax won't invoke the descriptor either.
Data descriptors (descriptors which implement __set__()) take precedence over instance attributes. If you changed LineDescriptor to implement __set__() as well as __get__(), it would not exhibit this behavior. But then you would have to figure out how to implement __set__(). That implementation will depend on what you are trying to accomplish:
You could raise AttributeError to indicate the descriptor is read-only. Then you'd have to remove the setattr() call, since you can't set a read-only descriptor.
You could implement __set__() to do the reverse of __get__(). Given the complexity of your __get__() method, I'm not entirely sure how to do that. It'll also change the effect of your setattr() call.
If you don't want to change or remove the setattr() call, you can always manually set an instance attribute:
self.__dict__[name] = attribute
This bypasses the data descriptor, and does exactly what the setattr() call is currently doing. However, the attribute probably won't match up with __get__()'s return value. In my opinion, that's poor design, but it's not strictly wrong.