When you compare two values using the is or is not operator, it is the object identities of the two values that is tested rather than their equality. If the class of either of the values in the comparison redefines equality then the is operator may return False even though the objects compare as equal.

CPython interns a number of commonly used values, such as small integers, which means that using is instead of == will work correctly. However, this might not be portable to other implementations such as PyPy, IronPython, Jython or MicroPython.

When you want to compare the value of two literals, use the comparison operator == or != in place of is or is not.

If the uniqueness property or performance are important then use an object that does not redefine equality.

The function equals_to_twelve() relies on CPython interning small integers.

To function correctly for all implementations, change the expression x is CONSTANT to x == CONSTANT.

Python Standard Library: Comparisons.

Stack Overflow: Python "is" operator behaves unexpectedly with integers.