Python - How to pass instance variable value to descriptor parameter?

Question

Considering below case,

ItemList Class:

# filename: item_list.py

# locators dictionary is intentionally placed outside `ItemList` class
locators = {
    'item_id': 'css=tr:nth-of-type({item_num}) .id',
    'item_title': 'css=tr:nth-of-type({item_num}) .alert-rules',
    # other properties
}

class ItemList(Item):
    # --- these are data descriptors ---
    # getting item_id value based on its item_num
    item_id = TextReadOnly(locators['item_id'].format(item_num=self.item_num))

    # getting item_title value based on its item_num
    item_title = TextReadOnly(locators['item_title'].format(item_num=self.item_num))


    def __init__(self, context, item_num=0):
        # self.item_num can't be passed to locators
        self.item_num = item_num

        super(ItemList, self).__init__(
            context
        )

In this case, I want to pass self.item_num value to item_num inside locators dictionary in ItemList class.

The reason is I want each of item_id and item_title refers to particular item_num.

I'm stuck on this part :

item_id = TextReadOnly(locators['item_id'].format(item_num=self.item_num))
item_title = TextReadOnly(locators['item_title'].format(item_num=self.item_num))

self.item_num value can't be passed to locators when instance is already made.

Implementation:

# filename: test.py

# print item_id values from item_num=1
item_1 = ItemList(context, 1)
print (‘Item_id_1: ’ + item_1.id)

# print item_id values from item_num=2
item_2 = ItemList(context, 2)
print (‘Item_id_2: ’ + item_2.id)

# ^ no result produced from item_1.id and item_2.id because self.item_num value can't be passed to locators

Is it possible to update data descriptor from instance variable?

How to pass instance variable value to data descriptor parameter?

Additional info:

I tried to call item_id and item_title as instance attribute but no luck. I noticed that data descriptor can’t be instance attribute from this post: Data descriptor should be a class attribute

Item Class:

# filename: item.py

import abc


class Item(Page):
    __metaclass__ = abc.ABCMeta

    def __init__(self, context):
        self.__context = context
        super(Item, self).__init__(
            self.__context.browser
        )

Descriptor:

# filename: text_read_only.py

class TextReadOnly(object):
    def __init__(self, locator):
        self.__locator = locator

    def __get__(self, instance, owner=None):
        try:
            e = instance.find_element_by_locator(self.__locator)
        except AttributeError:
            e = instance.browser.find_element_by_locator(self.__locator)

        return e.text

    def __set__(self, instance, value):
        pass

I think you are doing it wrong - the class attributes can't depend on instance attributes, as the class is initialized first. If you want `item_id` and `item_title` to vary per instance, why make them class attributes in the first place? — domoarigato, Aug 31 '17 at 09:12
Hello @domoarrigato: I make `item_id` and `item_title` as class attributes because I want to use descriptor **TextReadOnly**. I noticed from [this post](https://stackoverflow.com/questions/12599972/descriptors-as-instance-attributes-in-python) that data descriptor should be a class attribute. Is there another approach to implement it? — Raja David, Aug 31 '17 at 09:17
If you want to keep them as class attributes, you can try to make them as "protected class properties", see [this][1] question to see how. In your specific case I think this is overkill, `item_id` and `item_title` look more like Instance Descriptors than Data Descriptors, so I'll make them as instance attributes. [1]: https://stackoverflow.com/questions/5189699/how-to-make-a-class-property — Hrabal, Aug 31 '17 at 09:27
Hrabal and Domoarrigato: Thanks for both your answers. Those helped me to dig deeper about the concept of descriptor and instance attribute. I'll post the solution that worked for me. — Raja David, Sep 21 '17 at 16:53

score 0 · Accepted Answer · answered Sep 21 '17 at 17:14

I found this article and came up with solution to add the descriptors to an object.

This solution below worked for me:

Add __getattribute__ and __setattr__ into Item Class

# filename: item.py

import abc


class Item(Page):
    __metaclass__ = abc.ABCMeta

    def __init__(self, context):
        self.__context = context
        super(Item, self).__init__(
            self.__context.browser
        )

    def __getattribute__(self, name):
        value = object.__getattribute__(self, name)
        if hasattr(value, '__get__'):
            value = value.__get__(self, self.__class__)
        return value

    def __setattr__(self, name, value):
        try:
            obj = object.__getattribute__(self, name)
        except AttributeError:
            pass
        else:
            if hasattr(obj, '__set__'):
                return obj.__set__(self, value)
        return object.__setattr__(self, name, value)

Move class attribute to instance attribute in ItemList constructor

# filename: item_list.py

# locators dictionary is intentionally placed outside `ItemList` class
locators = {
    'item_id': 'css=tr:nth-of-type({item_num}) .id',
    'item_title': 'css=tr:nth-of-type({item_num}) .alert-rules',
    # other properties
}


class ItemList(Item):
    def __init__(self, context, item_num=0):
        self.item_num = item_num
        # self.item_num is able to be passed to locators
        self.item_id = TextReadOnly(locators['item_id'].format(item_num=self.item_num))
        self.item_title = TextReadOnly(locators['item_title'].format(item_num=self.item_num))

        super(ItemList, self).__init__(
            context
        )

Python - How to pass instance variable value to descriptor parameter?

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