Python Inconsistencies.ipynb

Oh yes, that one! It gets better though:

>>> def foo():
...     a = 1000
...     b = 1000
...     return a is b
... 
>>> foo()
True

The reason is that the constants used here are stored in the function object, where they're deduplicated:

>>> foo.__code__.co_consts
(None, 1000)

So inside the function both the literals are looked up as the same constant.

There is basically zero behaviour you can count on for when two numbers are going to be reference equal in Python.

Similar things are not always similar.

Python 3.4.3 (default, Nov 12 2015, 20:43:56) 
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a, b = b'foo', b'foo'
>>> a is b
True
>>> a, b = 'foo', 'foo'
>>> a is b
True
>>> a = 'foo'
>>> b = 'foo'
>>> a is b
True
>>> a = b'foo'
>>> b = b'foo'
>>> a is b
False