pyqt: dynamically update properties for dynamically created widgets

Question

I'm trying to find a way to update labels which are within dynamically created widgets (that can be deleted), according to properties dynamically set in the preceding widgets.

Is there a way to automatically and dynamically link a pyqt object to a property of other widgets, so that when I change a value anywhere, that object updates itself?

Example: object 'a' has property start, bars and end; bars is given, start is taken by the previous object (or 1 if None), end is calculated. object 'b' takes its start from a.end and so on.

class myclass(object):
  def __init__(self, referrer=None, value=1):
    self.referrer=referrer
    self.value=value

  def _get_start(self):
    if not self.referrer:
      return 1
    else:
      return self.referrer.end+1

  def _get_end(self):
    return self.start+self.value-1

  start=property(_get_start)
  end=property(_get_end)

def create(value=1):
  if not vList:
    ref=None
  else:
    ref=vList[-1]
  val=myclass(ref, value)
  vList.append(val)

def showList():
  for i in vList:
    print 'item: %d\tstart: %d\tend: %d' % (vList.index(i),i.start, i.end)

vList=[]

If I call create() 3 times, showList() will show:

item: 0 start: 1        end: 1
item: 1 start: 2        end: 2
item: 2 start: 3        end: 3

if I change vList[0].value to 3:

item: 0 start: 1        end: 3
item: 1 start: 4        end: 4
item: 2 start: 5        end: 5

The problem raises when I need to keep those values updated in the gui (think to it as an interface like this): every horizontal widget has a label showing the property of start, a spinbox for bars, and a label for end, and as soon as any spinbox value changes, every subsequent widget should update its start and end properties according to its previous widget and show them in the relative labels. Moreover, when any widget is deleted, all the subsequent widget should recompute every property.

Using getter/setter to set the values in the labels of the widgets' next instancies doesn't obviously work as I need, because when I change any x.value, the following instancies' start and end will be actually updated only when recalled, AFAIU. I could connect every new widget to its previous (with valueChanged()) or create a function which finds the subsequent widget and update their properties, but that's not a good solution.

Since I am almost new to python (most important: I'm not a programmer) I think that I am ignoring something about "connecting" variables in a better and cleanest way (maybe related to signals or threading?).

Consider that those widgets will actually be children widgets of another "main" widget, which will have similar properties: start taken from its previous main widget (if any), bars which is the sum of all bars in every children widget, end which will be again start+bars-a.

Thanks!

(I hope you will understand what I meant, my english is not perfect and, since I'm not a programmer, my terminology is not always correct)

Your question is detailed. It is very difficult to ascertain what you need help with. Try to distill this to a use case you are having trouble with, start with your question, then demonstrate an example. — Paul Seeb
– Paul Seeb, Commented Jan 30, 2013 at 18:23
I was afraid of that, sorry. I'm going to rewrite it better. Thank you. — musicamante
– musicamante, Commented Jan 30, 2013 at 18:33

reclosedev · Accepted Answer · 2013-02-04 05:28:58Z

I can't find use case for things from your question, but here is possible solution using Qt Signals-Slots:

# -*- coding: utf-8 -*-
import functools

from PyQt4 import QtGui, QtCore


class ObservableVariable(QtCore.QObject):
    """ Represents variable with value, when value changes it emits
    signal: changed(new_value)
    """
    changed = QtCore.pyqtSignal(object)

    def __init__(self, initial_value=0):
        super(ObservableVariable, self).__init__()
        self._value = initial_value

    def get_value(self):
        return self._value

    def set_value(self, new_val):
        self._value = new_val
        self.changed.emit(new_val)

    value = property(get_value, set_value)

    def __str__(self):
        return str(self.value)

    # it can support more operators if needed
    def __iadd__(self, other):
        self.value += other
        return self

    def __isub__(self, other):
        self.value -= other
        return self


class MyClass(object):
    def __init__(self, referrer=None, value=1):
        self.referrer = referrer
        self.value = ObservableVariable(value)
        self._initial_value = value
        if referrer:
            # propagate referrer changes to subscribers
            referrer.value.changed.connect(
                lambda x: self.value.changed.emit(self.value.value)
            )

    @property
    def start(self):
        if not self.referrer:
            return self.value.value
        return self.referrer.end + 1

    @property
    def end(self):
        return self.start + self.value.value - 1


class GuiExample(QtGui.QWidget):
    def __init__(self):
        super(GuiExample, self).__init__()
        self.values = []
        layout = QtGui.QVBoxLayout(self)
        obj = None
        for i in range(5):
            obj = MyClass(obj, i)
            self.values.append(obj)
            # create gui elements
            hlayout = QtGui.QHBoxLayout()
            spinbox = QtGui.QSpinBox()
            spinbox.setValue(obj.value.value)
            start_label = QtGui.QLabel()
            end_label = QtGui.QLabel()
            hlayout.addWidget(start_label)
            hlayout.addWidget(spinbox)
            hlayout.addWidget(end_label)
            layout.addLayout(hlayout)

            # function called on value change
            def update_start_end(instance, start_label, end_label):
                start_label.setText(str(instance.start))
                end_label.setText(str(instance.end))

            action = functools.partial(update_start_end, obj, start_label,
                                       end_label)
            action()  # set initial start end text to labels
            # connect signals to gui elements
            obj.value.changed.connect(action)
            spinbox.valueChanged.connect(obj.value.set_value)
            obj.value.changed.connect(spinbox.setValue)
        layout.addWidget(QtGui.QPushButton('test',
                                           clicked=self.test_modification))

    def test_modification(self):
        self.values[1].value += 1


app = QtGui.QApplication([])
gui = GuiExample()
gui.show()

app.exec_()

Wow, it looks like what I was looking for. It took a moment to understand how you assigned the referrer (didn't realize the assignment order of obj), but now I've understood everything. Now I've to decide if I want to assign every new referrer by hand (when I delete or insert widgets in the middle of the list), but maybe I can find a way to 'connect' them automatically using the values list. Thank you so much!

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