access class attributes from another without inheritance? use an array of classes as input

Question

I have a class:

class A():
 def __init__(self, a, b):
 self.a = a
 self.b = b
 def method_A(self):
 return self.a

Now, I have another class B:

class B():
 def __init__(self, c,d):
 self.c = c
 self.d = d
 def method_B(self):
 # do computations with a and b from class A
 # return A.a * c

One solution is to have B inherit from A but that means that it inherits all attributes of A which is something that I don't want.

For example it will be :

class B(A):
 def __init__(self,a,b,c,d)
 A().__init__(self,a,b)
 self.c = c
 self.d = d

So, I have to initialize my B class with def __init__(self,a,b,c,d) all arguments (from A and for B) , right?

Is there a way I can avoid that ?And still have access to class A attributes and methods?

---------- UPDATE 1 --------------------------------------

Updated according to the answers.

I want to be able to use an array of class A as input to class B.

I tried:

import numpy as np
class B():
 def __init__(self, c, d, the_a):
 self.c = c
 self.d = d
 self.the_a = the_a
 def method_B(self):
 for i in self.the_a:
 return self.the_a[0][0].a * self.c
a = np.array([[A(2, 4), A(3,5)]])
b = B(4, 4, a)
print(b.method_B())

and I am receiving : the value 8 .

But obvious I am not correctly using the loop in method_B.

I am using only the 0 index, but I can't figure!

------- UPDATE 2 ------------------------------

I am almost there..

 def method_B(self):
 for idx, x in np.ndenumerate(self.the_a):
 self.the_a[idx].a = self.the_a[idx].a * self.c
 self.the_a[idx].b = self.the_a[idx].b * self.c
 return self.the_a

Now, it returns me [[<__main__.A object at 0x7f165426c9b0> <__main__.A object at 0x7f165426c898>]] .

Is there a way I can receive the updated array like:

np.array([[ A(8, 16), A(12,20)]])

• Moses Koledoye: If you want to initialise an instance of A in B you're going to have to pass the arguments one way or another
• Steven Summers: You've given b a numpy array of A instances, do you want B to have multiple A? If not then just give B a single A instance instead of a list of them. Otherwise your B.__init__ needs to handle multiple A instances.
• George: @StevenSummers:yes, I want B to have as input an array of A instances.
• Steven Summers: self.the_aV1 = the_a.a and self.the_aV2 = the_a.b isn't going to work as the_a is a numpy array which as the error message states doesn't have an attribute a. What do you want the_aV1 and the_aV2 to be since you want multiple A?
• George: @StevenSummers:the a and b are the values of the A class.So, i want to extract these values from the input array.So, regarding a values , I have : 2 and 3.regarding b values , i have : 4 and 5.And I want to make calculations based on these values ( as in the method_B)

Answer 1

You need an instance of A:

def method_B(self, the_a):
 return the_a.a * self.c

• George: But how can i initialize the A instance?Don't I have to use the init parameters of A in the B class?Can you give me a more complete example please?

Answer 2

To follow on the previous answer, if you create an instance of A:

a = A(a, b)

then, in class B, you can pass in that instance as a single argument and access any method(or variable for that matter) by referring to it's instance:

class B():
 def __init__(self, c, d, instance_of_a):
 self.c = c
 self.f = d
 self.e = instance_of_a.[the method or variable you want from class A]

Hope this helps,

• George: And if I have many instances that I want to supply to B ?If I have an array(numpy) or list of A instances?
• Nick H: Ok, a list would of objects would be the way I would go here: link I don't think numpy can handle a list of objects.

Answer 3

Hold an instance of class A in class B.

class A():
 def __init__(self, a, b):
 self.a = a
 self.b = b
 def method_A(self):
 return self.a
class B():
 def __init__(self, c, d, the_a):
 self.c = c
 self.d = d
 self.the_a = the_a
 def method_B(self):
 return self.the_a.a * self.c
a = A(1, 2)
b = B(3, 4, a)

• George: And if I have many instances that I want to supply to B ?If I have an array(numpy) or list of A instances?
• Tobi S.: Instead of only one instance of A, you can pass a list with instances of A. In your method_B you can iterate over it and do whatever you nedd to do.
• George: Can you please give me an example?because , I am using something like : a = np.array([[A(2, 4), A(3,5)]]) and a loop in method_B but I have an error about accessing the attribute from class B init.