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
– domoarigato, Commented Aug 31, 2017 at 9:12
Hello @domoarrigato: I make item_id and item_title as class attributes because I want to use descriptor TextReadOnly. I noticed from this post that data descriptor should be a class attribute. Is there another approach to implement it? — Raja David
– Raja David, Commented Aug 31, 2017 at 9:17
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]: stackoverflow.com/questions/5189699/… — Hrabal
– Hrabal, Commented Aug 31, 2017 at 9:27
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
– Raja David, Commented Sep 21, 2017 at 16:53

Raja David · Accepted Answer · 2017-09-21 17:14:46Z

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 )

Collectives™ on Stack Overflow

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

1 Answer 1

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Linked

Related