How does python evaluate "is" expressions? [duplicate]

Question

Erratic behavior of "is" expressions in python.

>>> 258 -1 is 257
False

And

>>> 258 -1 == 257
True

1. How is python evaluating "is" expression ? and why does it show it as false , eventhough it is true ?

2. Why is it happening only to certain set of numbers ?

   2 - 1 is 1 True

works perfectly fine.

Answer 1

is is used for identity check, to check if both variables point to the same object, while == is used for checking values.

From the docs:

The operators is and is not test for object identity: x is y is true if and only if x and y are the same object. x is not y yields the inverse truth value.

>>> id(1000-1) == id(999)
False

""" What is id?
id(object) -> integer
Return the identity of an object. This is guaranteed to be unique among
simultaneously existing objects. (Hint: it's the object's memory address.)
"""

>>> 1000-1 is 999
False
>>> 1000-1 == 999
True
>>> x = [1]
>>> y = x    #now y and x both point to the same object
>>> y is x
True
>>> id(y) == id(x)
True
>>> x = [1]
>>> y = [1]
>>> x == y
True
>>> x is y
False
>>> id(x),id(y)       #different IDs
(161420364, 161420012) 

But some small integers(-5 to 256) and small strings are cached by Python: Why (0-6) is -6 = False?

#small strings
>>> x = "foo"
>>> y = "foo"
>>> x is y
True
>>> x == y
True
#huge string
>>> x = "foo"*1000
>>> y = "foo"*1000
>>> x is y
False
>>> x==y
True