Can I assign an object of a class to a variable if I already called it? Python 3

Question

class myClass:
    pass

myClass()

The use of myClass() creates an object of type myClass. How can I assign exactly that object to a variable after just calling it like that? Because if I add a = myClass() after the call to myClass(), another object will be created and a will point to that other object.

Answer 1

If you just call the class constructor and don't assign the result to a variable, no reference will be created. Not even the garbage collector knows of an instance of this class.

import gc

class myClass:
    pass

myClass() 
print( [o for o,t in globals().items() if isinstance(t, myClass)], [o for o in gc.get_objects() if isinstance(o, myClass)] )

a = myClass()
print( [o for o,t in globals().items() if isinstance(t, myClass)], [o for o in gc.get_objects() if isinstance(o, myClass)] )
del a

gc.disable() # even disabling the garbage collector will not make a difference
myClass()
print( [o for o,t in globals().items() if isinstance(t, myClass)], [o for o in gc.get_objects() if isinstance(o, myClass)] )

Output:

[] []
['a'] [<__main__.myClass object at 0x00000000046026D8>]
[] []

---

However, you could make the class keep track of references itself:

import gc

class myClass:
    ref = []
    def __init__(self):
        myClass.ref.append(self)

myClass()
print( [o for o,t in globals().items() if isinstance(t, myClass)], [o for o in gc.get_objects() if isinstance(o, myClass)] )

a = myClass()
print( [o for o,t in globals().items() if isinstance(t, myClass)], [o for o in gc.get_objects() if isinstance(o, myClass)] )
del a

gc.collect() # force garbage collection
print( [o for o,t in globals().items() if isinstance(t, myClass)], [o for o in gc.get_objects() if isinstance(o, myClass)] )

Output:

[] [<__main__.myClass object at 0x00000000048226D8>]
['a'] [<__main__.myClass object at 0x00000000048226D8>, <__main__.myClass object at 0x0000000004890860>]
[] [<__main__.myClass object at 0x0000000004890860>, <__main__.myClass object at 0x00000000048226D8>]

Note: this way, the garbage collector will never delete instances of this class, even if the variable is deleted, as long as the class keeps a reference to the instance!

Answer 2

class MyClass:
    instances = []

    def __init__(self, name: str):
        self.name = name

        MyClass.instances.append(self)

    def printName(self):
        print("My name is " + self.name)

    def setName(self, name: str):
        self.name = name


a = MyClass("Joshua")

a.printName()   # Outputs 'My name is Joshua'

b = a

b.printName() # Outputs 'My name is Joshua'

b.setName("DeepSpace")

a.printName()   # Outputs 'My name is DeepSpace'
b.printName()   # Outputs 'My name is DeepSpace'

MyClass.instances[0].printName()    # Outputs 'My name is DeepSpace'

When b = a is used, it means that b will point to the same memory location as a, this happens as your class is mutable. You could manually store instances to refer to later by storing them as a list in the base class.

Answer 3

To do something like this is impossible because of CPython garbage collection:

myClass()
a = lastObjectCreated()

But has no use case and is 100% impossible (at least with CPython) because the object is destroyed right after it is created sense no variable is pointing to it (no variable has a reference to the class).

---

A weak reference is a reference that does not keep a object from being deleted. With this, we can witness an object be deleted, like so:

class myClass:
    pass

import weakref

def catch_deletion(object):
    print('Object was deleted')

weakref.ref(myClass(), catch_deletion)
#Output: Object was deleted

That proves that it is truly gone.