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

问题

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

回答1:

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

This solution below worked for me:

1. 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)
   

2. 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
           )
   

来源：https://stackoverflow.com/questions/45977559/python-how-to-pass-instance-variable-value-to-descriptor-parameter