How can I overwrite an object in python? [duplicate]

Question

I have a program for which it would be most sensible to overwrite an object in memory instead of changing it, or redirecting a reference to it.

Basically, if I was writing C++, I would have multiple pointers all referring to the same address in memory. I would like to overwrite the contents at that address.

I would like to be able to do this from within the object itself as well.

i.e.

class MyClass
    def __init__(self):
        pass

    def overwrite(self,otherObjectOfTypeMyClass):
       #after this operation, all references to self now point to otherObjectOfTypeMyClass

Answer 1

Unlike C++, memory management is not under your control in Python. And in general, that is a good thing. Not having to worry about memory is one of the big benefits for those of us who started in C.

In python what we call variables are really labels referring to objects. So if you want to change an object that is not immutable (like a list), that should work out-of-the-box in Python.

However, you cannot really generally overwrite objects in Python. Creating a new object will use free memory. And memory that is in use will only be garbage-collected once it has no references to it.

Answer 2

OK, I've found a solution that seems to work for my specific usage. I don't know the internals of python well enough to say whether this would break in unexpected ways. I would be interested in hearing why or why not this would be a dangerous thing to do. I'd also be interested if anyone has any ideas of scenarios in which this would not do what I'm expecting it to. I will wait to mark this correct for a few days in case anyone thinks that it's actually catastrophic.

def replaceobject(obj1,obj2):
    #Replaces obj1 with obj2
    obj1.__dict__ = obj2.__dict__


class MyClass:
    def __init__(self):
        self.attr0 = None
        self.attr1 = None
        self.attr2 = None
        self.attr3 = None

    def set_some_attrs(self):
        self.attr0 = 0
        self.attr1 = "1"
        self.attr2 = {"2":2}
        self.attr3 = [3]

#Set up scenario, three objects of the same type, one points to a different address
A = MyClass()
A.set_some_attrs()
C = A
B = MyClass()

#Make replacement and spit contents
replaceobject(B,A)
print B.attr0 #=> 0
print B.attr1 #=> "1"
print B.attr2 #=> {"2":2}
print B.attr3 #=> [3]

#Modify the replaced object and changes are reflected
B.attr2 = "made a change"
print A.attr2 #=> "made a change"
print C.attr2 #=> "made a change"