deepcopy breaks the reference relationship in user-defined class

Question

I define a class, in which there is a reference relationship.

Then I create an instance, after copy.deepcopy this instance, the reference relationship is gone, for example:

import numpy as np
class foo(object):
    def __init__(self):
        self.c = np.array([[1,2],[3,4]])
        self.a = self.c[1,:]

ff0 = foo()
ff1 = copy.deepcopy(ff0)

ff1.c +=np.array([10,10])
print(ff1.a)

ff0.c +=np.array([10,10])
print(ff0.a)

Output:

[3 4]
[13 14]

But I want to see such a output:

[13 14]
[13 14]

Can anyone help me to preserve this relationship?

Thank you in advance~

Answer 1

I got a walk around to this problem:

import numpy as np
class foo(object):
    def __init__(self):
        self.c = np.array([[1,2],[3,4]])
        self.a = self.c[1,:]
    def copy(self):
        cp = copy.deepcopy(self)
        cp.a = cp.c[1,:]
        return cp

ff0 = foo()
ff1 = ff0.copy()

ff1.c +=np.array([10,10])
print(ff1.a)

ff0.c +=np.array([10,10])
print(ff0.a)

Output:

[13 14]
[13 14]