Initializing instance of class in another class constructor (python)

Question

I have a two different classes (A & B for example), and I'm trying to take an instance of class A as a parameter in class B. Edit: I'm trying to create the instance within the constructor of Class B so that I can use it for subsequent methods in Class B.

Basically this:

class A(object):
    def __init__(self, various arguments):
        self.a_arguments = arguments 

class B(object):
    def __init__(self, various arguments):
        self.b_arguments = arguments 
        self.a = A(arguments of A)

In this case, how can I list the "arguments of A" when trying to create the instance self.a? When I write the arguments as self.a_arguments, they can't be accessed because they are in Class A and not Class B. When I tried just a_arguments, I get an error that the a_arguments are not defined. Yet I don't want to have to rewrite all the self.a_arguments in the Class B constructor just so I can use them to create an instance of A.

Are you trying to take the A instance as a parameter, or create one? — user2357112
– user2357112, Commented Nov 1, 2017 at 17:27

Brandon Dube · Accepted Answer · 2017-11-01 18:23:07Z

If you want to pass some indeterminate number of arguments to something, you should use tuple unpacking. Here is some code:

class A(object):
    def __init__(self, *args):
        self.one, self.two, self.three_or_more = *args

class B(object):
    def __init__(self, arg_for_b, arg2_for_b, *a_args):
        self.ab = arg_for_b
        self.ab2 = arg2_for_b
        self.a = A(*a_args)

How does this work?

Class A takes any number of arguments via the splat (*) operator. Using tuple unpacking (*var), the fields of the class one, two, and three_or_more are assigned to the first, second, and remaining elements of the input arguments.

These could be passed as foo = A(1,2,3,4,5,6,7,8,9...inf), by putting *args in the constructor, there can be any number of args and they are not forced to be in a list or tuple.

B takes two arguments for itself, and the remaining arguments are splat'd into a set to pass to A.

In usage:

>>> from module import A,B
foo = B(1,2,3,4,5)

A note on design -- it is probably a bad idea to initialize another class inside one class constructor. It would probably be easier to understand if the code looked like this:

class A(object):
    def __init__(self, *args):
        self.one, self.two, self.three = *args

class B(object):
    def __init__(self, arg_for_b, arg2_for_b, A_obj):
        self.ab = arg_for_b
        self.ab2 = arg2_for_b,
        self.a = A_obj

>>> from module import A,B
foo = B(1,2,A(3,4,5))

eyllanesc · Accepted Answer · 2017-11-01 17:31:54Z

2

I would suggest providing the constructor for B an initialised object of type A, e.g.:

class B(object):
    def __init__(self, objectA):
        self.b_arguments = arguments 
        self.a = objectA

objectB = B(A(<args>))

edited Nov 1, 2017 at 17:31

eyllanesc

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answered Nov 1, 2017 at 17:30

japester

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