Python继承：从子类创建父类对象

Question

I have a set of classes where a child is derived from a parent. What I am trying to achieve is to create an object of the parent class that gets all its values from an object of the child class. The only way I found is this:

from copy import deepcopy
class Parent(object):
    def __init__(self, value):
        self.val = value

class Child(Parent):
    def __init__(self, val=8):
         super(Child, self).__init__(val)
         chval=5
par=Parent(3)
ch=Child()
parent= Parent(4)
parent.__dict__ = deepcopy(super(Child, ch).__dict__)
print(parent.val)
print(type(par), type(ch), type(parent))

The output is indeed

8
(<class '__main__.Parent'>, <class '__main__.Child'>, <class '__main__.Parent'>)

but I am not sure whether this is a good, pythonesque and risk-free method of doing this

Answer 1

Question : How would I create a Circle with the the basic Figure properties of a Rectangle ? """

You can do this by implementing a method new_from in the Base class Figure .
For example:

class Figure:
    def __init__(self, p):
        self.properties = p

    @classmethod
    def new_from(cls, obj):
        if issubclass(obj.__class__, Figure):
            _new = cls(obj.properties)
            return _new
        else:
            raise TypeError('Expected subclass of <class Figure>, got {}.'\
                                .format(type(obj)))

    def __repr__(self):
      return "<class '{}' properties:{}"\
                .format(self.__class__.__name__, self.properties)

class Rectangle(Figure):
    pass    

class Circle(Figure):
    pass

r1 = Rectangle({'test': 'r1.property'})
r2 = Rectangle.new_from(r1)
c1 = Circle({'test': 'c1.property'})
c2 = Circle.new_from(r1)

for obj in [r1, r2, c1, c2]:
    print(obj)  # '{}\n{}'.format(obj, obj.__dict__))

Output :

 <class 'Rectangle' properties:{'test': 'r1.property'} <class 'Rectangle' properties:{'test': 'r1.property'} <class 'Circle' properties:{'test': 'c1.property'} <class 'Circle' properties:{'test': 'r1.property'} 

Tested with Python: 3.6