The 'is' operator behave differently on two sets of values 
please find the attachment below

tested environment 
windows 7
python 3.7.3


Python 3.7.3 (v3.7.3:ef4ec6ed12, Mar 25 2019, 22:22:05) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
>>> a=256
>>> b=256
>>> a is b
True
>>> a=257
>>> b=257
>>> a is b
False

This will emit a SyntaxWarning in Python 3.8 to use == instead of using is for literals. This is not a bug but an implementation detail over caching a range of integers at https://github.com/python/cpython/blob/1f21eaa15e8a0d2b0f78d0e3f2b9e5b458eb0a70/Objects/longobject.c#L19

Further to Karthikeyan Singaravelan comment, the behaviour you see is absolutely correct. The operator isn't behaving differently, it is reporting precisely the truth.

The ``is`` operator tests for object identity, not equality. Python makes no promises about object identity of literals. If you use an immutable literal in two places:

    a = 1234
    b = 1234

the interpreter is free to use the same object for both a and b, or different objects. The only promise made is that ``a == b``.

The Python interpreter currently caches some small integers for re-use, but that's not a language guarantee, and is subject to change without warning. It has changed in the past, and could change again in the future.

The bottom line is that you shouldn't use ``is`` except to test for object identity, e.g. ``if obj is None``.