Removed a singleton comparison pitfall from the code

[NaelsonDouglas]

The problem In Python when comparing to singletons, the use of the operator 'is' instead of '==' is encouraged. This pitfall was detected using Pylint, under the code C0121 More about it can be found here: https://vald-phoenix.github.io/pylint-errors/plerr/errors/basic/C0121.html

Solution Replaced '==' to 'is' when comparing to singletons

[nuno-faria]

Thanks!