Object Model in Python - Understanding Internals

The object model of Python is something very less discussed but important to understand what happens under the cover. Understanding this before diving into python makes journey smooth

1. >>a = 5, What this line does is, it creates an object which holds value 5 & a refers to it.

2. Each Object consists of 3 things - data ( 5 in this case ), type ( int in this case ) & reference_count ( 1 in this case)

3. If we allocate numbers below 255 or string of smaller size ( not separated by special characters ), Python Object Model shares the object by increasing the reference_count.

4. If two references a & b refers to same objects, id(a) == id(b) will evaluate to be true. Another way to test this is a is b.

5. a = 5, b = 5. Reference count of the object containing 5 is 2 now.

6. a = 6. Reference count of object containing 5 is one now

7. b = 7. Reference count of object containing 5 is zero now.

8. Objects are eligible for memory cleanup when their reference count is zero. Periodic Garbage collection will reclaim the memory 

9. One important point to note here is, numbers above 255 & longs strings are not shared because chances of them using in code is far less. Doing a lookup everytime will impact CPU utilization every time a variable is assigned. This provides an equilibrium in memory vs CPU utilization.

10. Because of the type information associated with every object, 5 & 5.0 will occupy a different piece of memory. And, hence different objects.