python object references

points:

• variables only refer to object
• == and is
• copy and deepcopy
• function parameters are passed as aliases
• del and reference counting

Variables Are Not Boxes

Variables are not boxes, it’s better to think of them as labels attached to objects.

>>> class Gizmo:
...     def __init__(self):
...         print("Grimo id: {}".format(id(self)))

>>> x = Gizme() * 10
# Gizmo id: 4301489432
# TypeError: unsupported operand type(s) for *: 'Gizmo' and 'int'

>>> dir()
# ['Gizmo', '__builtins__', '__doc__', '__loader__', '__name__', '__package__', '__spec__']

Variables are assigned to objects only after the objects are created.

Identity, Equality, and Aliases

charles and lewis refer to the same object

>>> charles = {'name': 'Charles L. Dodgson', 'born': 1832}
>>> lewis = charles
>>> charles == lewis
True
>>> id(charles) == id(lewis)
True
>>> charles is lewis
True
>>> lewis["balance"] = 950
>>> charles
{'name': 'Charles L. Dodgson', 'balance': 950, 'born': 1832}

alex and charles compare equal, byt alex is not charles

>>> alex = {'name': 'Charles L. Dodgson', 'born': 1832, 'balance': 950}
>>> alex == charles # 1
True
>>> alex is not charles
True

1. alex and charles compare equal, because of the __eq__ implementation in the dict class

Every object has an identity, a type and a value. An object’s identity never changes once it has been created; you may think of it as the object’s address in memory. The is operator compares the identity of two objects.

— The Python Language Reference

Function Parameters as References

The paremeters inside the function become aliases of the actual arguments. The result of this scheme is that a function may change any mutable object passed as a parameter.

>>> def f(a, b):
...     a+=b
...     return a
>>> a = [1, 2]
>>> b = [3, 4]
>>> f(a, b)
>>> (a, b)
([1, 2, 3, 4], [3, 4])

Mutable Types as Parameter Defaults: Bad Idea

The default values are only evaluated at the point of function definition in the defining scope.

defensive programming with mutable parameters

class Defensive:
    def __init__(self, para=None):
        if para is None:
            self.para = []
        else:
            self.para = list(para) # 1

1. list(para) produces a shallow copy or convert it to a list

del and Garbage Collection

Objects are never explicitly destroyed; however, when they become unreachable they may be garbage-collected.

— The Python Language Reference

The del statement deletes names, not objects.

In CPython, the primary algorithm for garbage collection is reference counting. Essentially, each object keeps count of how many references point to it. As soon as that refcount reaches zero, CPython calls the __del__ method on the object which should not be called by your code.