Receiving value from QgsPropertyOverrideButton if data defined override is checked

Question

I am trying to develop a plugin for QGIS 3. For me it is quite the first time really working with pyqgis or python, so there might be something simple I am missing.

I want to use data defined override button within this plugin. If it is checked, the used value should be taken from this button (expression or attribute) instead of a spinbox. This is my layout with the button:

Unfortunately I could not find any documentation or related questions, which explain how to actually use this QgsPropertyOverrideButton. So I am only able to grab the value from the spinbox. This is my relevant code snipped:

    #Check where to gather attributes from: GUI or Layer?
    if self.dlg.Isochrones_WalkSpeed_Override.isActive() == True:
        WalkSpeed = self.dlg.Isochrones_WalkSpeed_Override.value() #Receiving Value from GUI: DataDefinedOverride
    else:            
        WalkSpeed = self.dlg.Isochrones_WalkSpeed.value() #Receiving Value from GUI: SpinBox

How do I grab the value from there either via an attribute from layer or via a given expression within edit field?

Answer 1

I think, you should use toProperty() method, which returns QgsProperty. Then you can use value() or valueAs***() methods of QgsProperty to get value of defined expression. Or you can get raw expression string with expressionString() method. See docs for QgsProperty.

EDIT

value() and valueAs***() returns tuple where first element is actual value and second element is bool reffering to successfulness of conversion. So, it is possible to use python tuple unpacking feature like so:

val, success = property.value(context)
if success:
    doSomenthingWith(val)

So in your case you would need to write as example:

WalkSpeed, success = self.dlg.Isochrones_WalkSpeed_Override.toProperty().value(QgsExpressionContext())