When does Python Create a New Object

Question

Python uses names to reference objects. When we say a = b, a and b now refer to the same object, and if we change a, we will see the same change in b. For example:

a = [1, 2, 3]
b = a

a.append(4)

print(b)

will print [1, 2, 3, 4].

However, some operations do create new objects. For example:

a = 1
b = a

a = a + 1

print(b)

will print 1. Clearly the line a = a + 1 somehow creates a new object with a value of a + 1 and binds the name a to it. How does this work? What is the mechanism that creates a new object in this case?

This question is not a duplicate of this one, as I am asking specifically how the object creation happens in this case, whereas the other question was more generally about when names share objects.

Answer 1

I found the information I was looking for on the mechanics of object creation in Luciano Ramalho's book Fluent Python.

Ramalho explains that while __init__ is usually considered the "constructor", in fact, objects are created by calling the __new__ method. It is a class method, and it returns an instance of a class, which gets passed to __init__ as the first argument (conventionally "self"). Thus, __init__ is the initializer, not the constructor.

This can be illustrated with the following example:

class C(object):
    def __new__(cls):
        new_inst = object.__new__(cls)
        print new_inst.__dict__
        new_inst.var_1 = 1
        return new_inst

    def __init__(self):
        print self.__dict__
        self.var_2 = 2

a = C()             # Prints:
                        # {}
                        # {'var_1': 1}
print a.__dict__    # returns {'var_2': 2, 'var_1': 1}

b = C.__new__(C)    # Prints:
                        # {}
print b.__dict__    # returns {'var_1': 1}

b.__init__()        # Prints:
                        # {'var_2': 2, 'var_1': 1}
print b.__dict__    # returns {'var_2': 2, 'var_1': 1}

The line a = C() calls __new__ followed by __init__, as we can see from the printouts of __dict__. This can be broken into separate calls, as shown with the object b.

This question is also relevant.