在Python中将类分配给没有依赖性的变量

Question

I saw similar question here on stackoverflow but it wasn't satisfactorily answered. (link to this question - How to copy a python class? )

So my question is, is there any possible way to create a copy of class (instance of a class), that is stored in variable A and assign this copy of class to new variable B without having these variables A and B mutually dependent?

To be clear - is there a quite simple way?

Model situation would be:

class B:
    def __init__(self, li):
        self.l = li * 5
        self.u = 5       

class A:
    def __init__(self):
        self.x = [1, 2, 3]
        self.c = B([45, 1, 3])
    self.i = 1

x = A()
y = x #This copies only reference to x

Answer 1

Implement __copy__() for your class, then you can use copy.copy() to create a copy of the instance. This can't be done automatically due to the nature of Python objects (there is no way of knowing what variables will be set on an instance, and no way of knowing how it's been mutated, it's up to you to ensure that data is copied correctly).

Normally, presuming a class that functions in a normal way, __copy__() will probably be something simple like this for a supposed class SomeClass :

def __copy__(self):
    return SomeClass(self.somefield, self.otherfield, self.somethingelse)

Naturally, exact implementation will vary wildly based on the class itself.

Answer 2

如果要创建对象的单独副本，请尝试使用copy模块 。

Answer 3

Using deepcopy in copy module is simpler. Erm, guys here's solution to those subtle problems: http://docs.python.org/2/library/copy.html#copy.deepcopy