Inheritance without inheriting, is there create dinamically attributes that point to other class variables?

Question

I'm struggling to solve with this problem.

I'd like to have the name variable to be like a pointer to the value of self.model.name. If the value of _class.model.name is changed the _class.name should change accordingly.

Can't find a way to basically map dynamically the Class attributes with any Model attributes without inheriting.

class Model(object):

    name = 'foo'
    parent = 'foo_p'

class Class(object):

    model_class = Model

    def __init__(self):

        self.model = self.model_class() 

        setattr(self, 'name', self.model.name)

_class = Class()
print _class.model.name # foo

_class.model.name = 'foo_1'        
print _class.name # this should be foo_1

Thanks!

MisterMiyagi · Accepted Answer · 2018-10-22T11:54:08.900

1

Use a property to create a single dynamically computed attributes:

class Class(object):
    _model_class = Model

    @property
    def name(self):
        return Class._model_class.name

This causes all instances of Class to run the name method whenever the attribute name is looked up. This allows the value to be dynamically computed on each lookup.

_class = Class()
print(_class.name)  # 'foo'
Model.name = 'bar'
print(_class.name)  # 'bar'

If you want to dynamically fetch many or all attributes from somewhere else, consider using __getattr__:

class Class(object):
    _model_class = Model

    def __getattr__(self, name):
        # name is a *string* containing the name of the attribute to fetch
        return getattr(Class._model_class, name)

The __getattr__ is only triggered for attributes that are not on the class/instance. This makes it rather straightforward to use with manually defined attributes. Note that you can use arbitrary code to restrict what is fetched - e.g. raise AttributeError if name is not in some whitelist.

edited Oct 22 '18 at 11:54

answered Oct 19 '18 at 16:17

MisterMiyagi

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How can I create dynamically properties based on the Model attributes? – Adriano Muraca Oct 19 '18 at 16:23
@AdrianoMuraca The example code already shows a dynamic property using the ``Model`` attributes. Please clarify what you need. – MisterMiyagi Oct 20 '18 at 10:43
Thank you for your time @MisterMiyagi! :) this is what I meant for dynamically, don't know if it makes sense. Inside the __init__ function of the Class, I was thinking to add something like `for attribute in any_attribute_found_in_Model: set property in this class` In this way I wouldn't need to write manually the property any time a new attribute is added to the Model class. – Adriano Muraca Oct 22 '18 at 09:40
@AdrianoMuraca I have added a different approach that is suitable for generically fetching attributes based on rules. – MisterMiyagi Oct 22 '18 at 11:55
1

It was exactly what I was looking for. Thanks again. – Adriano Muraca Oct 22 '18 at 13:33

score 1 · Answer 2 · answered Oct 19 '18 at 16:24

Along with MisterMiyagi's answer - but in case you want to still want to keep the concerns separated (even though it really doesn't seem like you do)

class Class(object):
    model_class = Model

    def __init__(self):
        self.model = self.model_class() 
        setattr(self, 'name', self.model.name)

    # This is what we'll be changing
    @property
    def model_name(self):
        return self.model.name

    # This method will get called whenever we change the model_name
    @model_name.setter
    def model_name(self, new_name):
        self.model.name = new_name
        self.name = new_name

_class = Class()
_class.model_name # foo

_class.model_name = "bar" # both _class.model.name == bar and _class.name == bar now

Inheritance without inheriting, is there create dinamically attributes that point to other class variables?

2 Answers2