apply decorator to a class attribute only

Question

not sure if it's possible, but I would like to replicate the idea of @annotations from java in a python class. The goal would be to iterate through all attributes in this class and return those that are "marked" with a custom decorator (e.g. @render). So, in this case, I can define a generic function that renders a generic UI/view template to display only the render attributes from any class.

In this imaginary code, the Item class would be:

class Item:
    def __init__(self):
        self.id = 100
        @render
        self.desc = 'info about'
        @render
        self.title = 'product x'
        self.vender_ref = 'af2k102hv813'

and only title and desc would be shown/returned. What would be the way to achieve this feature? probably I'm way too far from pythonic thinking here, so appreciate any ideas.

If you say 'attributes' you strictly mean attributes, but not methods, yes? As far as I know, you can only decorate methods, with the exceptions of `properties` of course. — mapf, May 11 '20 at 21:28

score 1 · Accepted Answer · answered May 11 '20 at 21:27

1

I don't think it's possible to decorate attributes the way you are doing it here. One solution could be this.

class Item:
    def __init__(self):
        self.id = 100
        self.desc = "info about"
        self.title = "product x"
        self.vender_ref = "af2k102hv813"
        self.__rendered_attributes__ = ["desc", "title"]


def render_item(i):
    for attribute in i.__rendered_attributes__:
        value = getattr(i, attribute)
        print("attr", attribute, "=", value)


i = Item()
render_item(i)

answered May 11 '20 at 21:27

valentin

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Why? `render_item` works for any class that implements the `__rendered_attributes__` list of attributes – valentin May 11 '20 at 21:36
1

Ok, I see it, render is outside the class, it could be located in the view template – Savrige May 11 '20 at 21:40

mapf · Answer 2 · 2020-05-12T07:39:35.423

You could use something similar as described in this answer and name your attributes that you want to decorate in a specific way. This may be a bit extreme though. I think @valentin's approach is better, but I just wanted to throw another idea out there. Maybe it can be improved upon.

import inspect


class MyClass(object):
    at_a = '12'
    at_b = '34'

    def __init__(self):
        self.at_c = 'test'
        self.get_attr()

    def get_attr(self):
        attributes = inspect.getmembers(
            self, lambda a: not (inspect.isroutine(a))
        )
        attributes = [a for a in attributes if a[0].startswith('at_')]
        print(attributes)

    def myfunc(self):
        return self.at_a


test = MyClass()

Here is another way, making use of redefining the __setattr__ method. This one doesn't work with class attributes though:

class MyClass(object):
    a = '@render', '12'
    b = '34'

    def __init__(self):
        super().__setattr__('__rendered_attributes__', {})
        self.id = 100
        self.desc = '@render', "info about"
        self.title = "product x"
        self.vender_ref = "af2k102hv813"

    def myfunc(self):
        return self.a

    def __setattr__(self, key, value):
        if isinstance(value, tuple) and value[0] == '@render':
            self.__rendered_attributes__[key] = value[-1]
            value = value[-1]
        super().__setattr__(key, value)


test = MyClass()
print(test.__rendered_attributes__, test.a)

apply decorator to a class attribute only

2 Answers2