Variables, Objects and References¶

Variables are created when assigned, can reference any type of objects, and must be assigned before they are referenced.

Variable: Entires in a system table, with spaces for links to objects
Object: Pieces of allocated memory, with enough space to represent the values for which they stand.
References: Automatically followed pointers from variables to objects.
- Python references are like C "void*" pointer which are automatically followed whenever used

Types live with objects, not vaiables.

In [1]:

a = 3

In the assignment above, python do the following three steps:

In [2]:

x = 123
x = "abc"
x = 123.456

Each time x is assign to a new object, Python reclaims the prior objects's space.
Python will clean up unused space as program runs.

Variables are always pointers to objects, not labels of changeable memory areas.
But, Python's mutable types do in-place object changes.

In [3]:

list1 = [1, 2, 3]
list2 = list1

list1[0] = 4

print(list1)
print(list2)

[4, 2, 3]
[4, 2, 3]

The behavior above only occurs for mutable objects that supoort in-place changes.

In [4]:

list3 = [1, 2, 3]
list4 = list3[:]
list3[0] = 4

print(list3)
print(list4)

[4, 2, 3]
[1, 2, 3]

The method above support only list type.
The following method works for all mutable types.

In [5]:

import copy

dict1 = {"a": 1, "b": 2}
dict2 = copy.deepcopy(dict1)

dict1["a"] = 3

print(dict1)
print(dict2)

{'b': 2, 'a': 3}
{'b': 2, 'a': 1}

The difference between == operator and is operator¶

In [6]:

list1 = [1, 2, 3]
list2 = list1

print("list1 == list2  :", list1 == list2)
print("list1 is list2  :", list1 is list2)

list1 == list2  : True
list1 is list2  : True

In [7]:

list1 = [1, 2, 3]
list2 = [1, 2, 3]

print("list1 == list2  :", list1 == list2)
print("list1 is list2  :", list1 is list2)

list1 == list2  : True
list1 is list2  : False

Dynamic typing is the root of Python's polymorphism