how to use instance attribute to define a class attribute?

Question

I'm trying to use the name in my init for my class attribute, attr but it seems that's impossible. here's the code:

class B:

    def __init__(self, val):
        self.val = val

    def __get__(self, instance, owner):
        return owner.valEditNew(self.val)

    def __set__(self, instance, value):
        return


class A:

    def __init__(self, name = 'def_name'):
        self.name = name

    attr = B('G120')

    def valEditNew(val):
        val += ' @edited'
        return val

a = A('JJ')
print(a.attr)

that's it if i use self.name or name or ... in place of G120>>> builtins.NameError: name 'self' is not defined if that's not possible, can you show me the way?

falsetru · Accepted Answer · 2017-03-12 12:26:21Z

1

If you want to access attribute of the instance that contains the descriptor object, use instance parameter of __get__ / __set__:

class B:

    def __get__(self, instance, owner):
        return instance.valEditNew(instance.name)  # <---

    def __set__(self, instance, value):
        return


class A:

    attr = B()

    def __init__(self, name='def_name'):
        self.name = name

    def valEditNew(self, val):
        val += ' @edited'
        return val


a = A('JJ')
print(a.attr)
# prints => JJ @edited

answered Mar 12, 2017 at 12:26

falsetru

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3 Comments

shiyon sufa Over a year ago

I know that I'm overriding get and set methods here... where can I find the original ones ?

falsetru Over a year ago

@shiyonsufa, I recommend you to read docs.python.org/3/howto/descriptor.html or docs.python.org/3/reference/datamodel.html#descriptors

falsetru Over a year ago

@shiyonsufa, B is not subclass of any class, but object which does not have __get__, __set__; no overriding is done. Try object.__get__ in interactive shell.

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