[WIP] codebase refactor.

[wiseaidev]

The following steps were performed:

• Removed unnecessary keys method call.
• Replaced the filter method with a list comprehension to increase readability.
• ~~Replaced old python2 methods with their equivalent in python3 (e.g. xrange -> range, izip -> zip)~~ didn't notice the use of six module.
• Used f-strings whenever possible because it is the ultimate method for string formatting.
• Removed unnecessary list syntax from the sorted method because it takes an iterable which is in this case a generator.
• Removed unnecessary boolean literals.
• ~~Found and fixed a syntax error: typ is not str -> not isinstance(typ, str)~~.
• Replaced None not in with all.

Question: Why the cache size is set to None? This can become problematic as explained here.