A literal string type for Python

The problem with taint checking is that experience has shown that - even if it's always correct, which it often isn't - it leads a surprising number of programmers to assume that because the evil bit isn't set, then it must therefore be good. In other words, taint checking separates data into "definitely unsafe" and "might be safe assuming you're using it correctly, whatever that might mean", whereas many developers treat is as meaning "maybe unsafe" versus "definitely safe".

By restricting the feature to simply "is this a literal string, or derived from literal strings purely by means of concatenation"[0], the meaning is well-defined and easier to understand. In other words, it depends less upon programmer education, which is a strategy that has been repeatedly proven ineffective.

There is some discussion about this in https://wiki.php.net/rfc/is_literal if you're interested - that's the proposal for a very similar feature in PHP, which sadly did not pass for reasons I've not yet investigated.

[0] This PEP is a bit broader than that and does include some operations that create substrings, which makes me uncomfortable.