Python assignment to self in constructor does not make object the same

Question

I am making a constructor in Python. When called with an existing object as its input, it should set the \"new\" object to that same object. Here is a 10 line demonstratio

Answer 1

self, like any other argument, is among the local variables of a function or method. Assignment to the bare name of a local variable never affects anything outside of that function or method, it just locally rebinds that name.

As a comment rightly suggests, it's unclear why you wouldn't just do

b = a

Assuming you have a sound reason, what you need to override is not __init__, but rather __new__ (then take some precaution in __init__ to avoid double initialization). It's not an obvious course so I'll wait for you to explain what exactly you're trying to accomplish.

Added: having clarified the need I agree with the OP that a factory function (ideally, I suggest, as a class method) is better -- and clearer than __new__, which would work (it is a class method after all) but in a less-sharply-clear way.

So, I would code as follows:

class A(object):

    @classmethod
    def make(cls, value):
        if isinstance(value, cls): return value
        return cls(value)

    def __init__(self, value):
        self.attribute = value

Now,

a = A.make(1)
b = A.make(a)

accomplishes the OP's desires, polymorphically over the type of argument passed to A.make.