如何仅获取父类对象的属性

Question

I have 2 classes:

class Parent(object):
    def __init__(self, id, name):
        self.id = id
        self.name = name
        self.__parent_vars = ['id', 'name']  # make a copy

    def print_values(self):
        res = {}
        for el in self.__parent_vars:
            res[el] = vars(self)[el]
        return res


class Child(Parent):
    def __init__(self, id, name, last_name, age):
        Parent.__init__(self, id, name)
        self.last_name = last_name
        self.age = age

What I want to do - is to get from Child parameters of Parent class. I made it using additional variable and it works, but I need more elegant solution without additional variable. I need it for pickle class. If I create additional variable it breaks my Schemas in a big project.

I try to find something like this:

c = Child(12,"Foo","whatever",34)
vars(c.super())

with expected output:

{'id': 12, 'name': 'Foo'}

---

I found this question: Get attributibutes in only base class (Python) but it has significant difference of mine, so I can't use that solution.

Answer 1

I am afraid you cannot easily. In Python, classes only carry methods and static attributes. Non static attributes are commonly stored in the __dict__ attribute of the objects. That means that except in special cases, you cannot easily know which attributes where assigned in a parent class method, in a child class method or even outside any method.

I can only imagine a meta_class that would instrument the __init__ method to store which attributes were changed during its call:

import collections
import functools
import inspect

class Meta_attr(type):
    init_func = {}
    attrs = collections.defaultdict(set)

    def __new__(cls, name, bases, namespace, **kwds):
        c = type.__new__(cls, name, bases, namespace, **kwds)
        cls.init_func[c] = c.__init__
        @functools.wraps(c.__init__)
        def init(self, *args, **kwargs):
            before = set(self.__dict__.keys())
            cls.init_func[c](self, *args, **kwargs)
            after = set(self.__dict__.keys())
            cls.attrs[c].update(after.difference(before))
        init.__signature__ = inspect.signature(c.__init__)
        c.__init__ = init
        return c

class Parent(object, metaclass=Meta_attr):
    def __init__(self, id, name):
        self.id = id
        self.name = name

    def print_values(self):
        res = {}
        for el in Meta_attr.attrs[Parent]:
            res[el] = vars(self)[el]
        return res


class Child(Parent):
    def __init__(self, id, name, last_name, age):
        Parent.__init__(self, id, name)
        self.last_name = last_name
        self.age = age

It gives:

>>> c = Child(1,"a","b", 20)
>>> c.print_values()
{'id': 1, 'name': 'a'}

BEWARE : if an attribute is set outside of the __init__ method, it will not be registered by this meta class...

Answer 2

I found one more solution. It looks simplier, so I used it. It based on using Marshmallow Schemas. Also It is more usiful in my project because I already used marshmallow Schemas. The idea: I create 2 classes and 2 different Schemas. And can serialize bigger class (Child) using just Parent schema:

Code

Define 2 classes:

class Parent(object):
    def __init__(self):
        self.id = 'ID'
        self.name = 'some_name'

class Child(Parent):
    def __init__(self):
        Parent.__init__(self)
        self.last_name = 'last_name'
        self.age = 15

from marshmallow import Schema, fields, EXCLUDE

Define 2 Schemas:

class ParentSchema(Schema):
    ba = Parent()
    id = fields.Str(missing=ba.id)
    name = fields.Str(missing=ba.name)

class ChildSchema(Schema):
    ba = Child()
    id = fields.Str(missing=ba.id)
    name = fields.Str(missing=ba.name)

    last_name = fields.Str(missing=ba.last_name)
    age = fields.Int(missing=ba.age)

use it to get only Parent attributes for Child object:

user_data = {'id':'IDIDID', 'name':'add_name', 'last_name':'FAMILIYA'}
res = ParentSchema().load(user_data, unknown=EXCLUDE)

# output:
res
{'name': 'add_name', 'id': 'IDIDID'}