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

The only way to make it work exactly as you have it is to implement __new__, the constructor, rather than __init__, the initialiser (the behaviour can get rather complex if both are implemented). It would also be wise to implement __eq__ for equality comparison, although this will fall back to identity comparison. For example:

>>> class A(object):
    def __new__(cls, value):
        if isinstance(value, cls):
            return value
        inst = super(A, cls).__new__(cls)
        inst.attribute = value
        return inst
    def __eq__(self, other):
        return self.attribute == other.attribute


>>> a = A(1)
>>> b = A(a)
>>> a is b
True
>>> a == b
True
>>> a == A(1)
True  # also equal to other instance with same attribute value

You should have a look at the data model documentation, which explains the various "magic methods" available and what they do. See e.g. __new__.