I want to know how Python knows (if it knows) if a value-type object is already stored in its memory (and also knows where it is).

For this code, when assigning the value 1 for b, how does it know that the value 1 is already in its memory and stores its reference in b?

>>> a = 1

>>> b = 1

>>> a is b

True

Python uses shared small integers to help quick access. Integers range from [-5, 256] already exists in memory, so if you check the address, they are the same. However, for larger integers, it's not true.

a = 10e5

b = 10e5

a is b # False

Wait, what? If you check the address of the numbers, you'll find something interesting:

a = 1

b = 1

id(a) # 4463034512

id(b) # 4463034512



a = 257

b = 257

id(a) # 4642585200

id(b) # 4642585712

It's called integer cache. You can read more about the integer cache here: https://wsvincent.com/python-wat-integer-cache/

“The current implementation keeps an array of integer objects for all
integers between -5 and 256, when you create an int in that range you
actually just get back a reference to the existing object.”

Why? Because small integers are more frequently used by loops. Using reference to existing objects instead of creating a new object saves an overhead.

If you take a look at Objects/longobject.c, which implements the int type for CPython, you will see that the numbers between -5 (NSMALLNEGINTS) and 256 (NSMALLPOSINTS - 1) are pre-allocated and cached. This is done to avoid the penalty of allocating multiple unnecessary objects for the most commonly used integers. This works because integers are immutable: you don't need multiple references to represent the same number.

Python doesn't know anything until you tell it. So in your code above, when you initialize a and b, you are storing those values(in the register or RAM), and calling the place to store it a and b, so that you can reference them later. If you didn't initialize the variable first, python would just give you an error.

Why?

is is kinda confusing:

@ajnLJA-0184 is correct, but additionally, strings work well too, but when you do some operations to it, but still the same strings, nope:

>>> a = 'python'

>>> b = 'python'

>>> a is b

True

>>> a = 'pytho' + 'n'

>>> b = 'pythonn'[:-1]

>>> a is b

False

>>> 

But strange enough, it is different for integers:

>>> a = 1

>>> b = 1

>>> a is b

True

>>> a = 3*8

>>> b = 4*6

>>> a is b

True

>>> 

But as the link he gave, there you can see that:

>>> a = 257

>>> b = 257

>>> a is b

False

>>> 

How to know if it is gonna be True or not?

Well, here is when id comes in handy:

Here you go, just type in id and two parens and say your varaible...:

>>> a = 1

>>> b = 1

>>> id(a)

1935522256

>>> id(b)

1935522256

>>> a = 257 # or -6

>>> b = 257 # or -6

>>> id(a)

935705330960

>>> id(b)

935705331216

>>>