[FIXED] How to fetch class instance from class variable (from outside the class)?

Issue

Let's say I have this:

from PySide2 import QtWidgets

class MyClass(object):
    def __init__(self, parent=None):
        self.class_variable = QtWidgets.QWidget()

class_instance = MyClass()
variable = class_instance.class_variable
class_instance_returned = mystery_method(variable)  # How to make this return class_instance?

How should I define mystery_method so that it would return the class_instance instance?

The real-world case I have is that I'm sending a QWidget which I'm using as a base instance for .ui file loading into a function. Inside this function I need to figure out which class instance it belongs to.

Solution

One way would we to define foo as a custom property that returns both its value and the related instance when its value is fetched:

from collections import namedtuple


class Prop(object):
    def __init__(self, val):
        self.val = val

    def __get__(self, instance, type):
        return namedtuple('Prop', ('value', 'instance'))(self.val, instance)

    def __set__(self, instance, val):
        self.val = val


class MyClass(object):
    foo = Prop('bar')

Now in your program you can explicitly use its value and the related instance using foo's value and instance attributes respectively.

Demo:

>>> instance = MyClass()
>>> instance.foo
Prop(value='bar', instance=<__main__.MyClass object at 0x10effbcd0>)
>>> instance.foo.value
'bar'
>>> instance.foo.instance
<__main__.MyClass object at 0x10effbcd0>

Answered By - Ashwini Chaudhary