Notebook's TaggingRules inherit too much

Question

Bug introduced in V10.4 or earlier and persisting through V12.0

[CASE:3852725] was created

[...] I have forwarded an incident report to our developers with the information you provided. [...]

---

This small helper function will show us TaggingRules written explicitly to the notebook. (Because Options/CurrentValue show values merged from $FrontEnd/$FrontEndSession too)

nbTR[] :=  TaggingRules /. List @@ Rest @ NotebookGet @ EvaluationNotebook[]

Here is the story:

• create a new notebook and evaluate:

  CurrentValue[$FrontEndSession, {TaggingRules, "a"}] = 1;
  

• now we can check that notebook's TaggingRules are nicely merged even though they are not written explicitly:

  CurrentValue[EvaluationNotebook[], {TaggingRules, "a"}]
  nbTR[]
  

  1

  TaggingRules

• Pay attention now! Let's use example from documentation: TaggingRules / Applications

  So we want to have some tagging rules local to the notebook:

  CurrentValue[
    EvaluationNotebook[], {TaggingRules, "InputFieldState"}
  ] = "initial string";
  

• And let's check:

   CurrentValue[EvaluationNotebook[], {TaggingRules, "InputFieldState"}]
  

  "initial string"

  but

  nbTR[]
  

  {......., "a" -> 1 (??!!), "InputFieldState" -> "initial string"}

So the bottom line is, if the notebook doesn't contain TaggingRules explicitly then it will inherit all you have in $FrontEnd/$FrontEndSession!

And this is a problem because:

• I didn't modify "a"'s value on a notebook level but it is now localized here and won't inherit external changes:

  CurrentValue[$FrontEndSession, {TaggingRules, "a"}] = 22;
      CurrentValue[$FrontEndSession, {TaggingRules, "a"}]
  CurrentValue[EvaluationNotebook[], {TaggingRules, "a"}]
  

  22

  1 (!!!)

• Not to mention that you write to file possibly private data without notice.

I consider this a serious flaw. Any suggested prevention measures?

Answer 1

The only solution I've found is to set empty Tagging rules first:

CurrentValue[EvaluationNotebook[], TaggingRules] = {};

CurrentValue[ EvaluationNotebook[], {TaggingRules, "InputFieldState"}] = "initial string";

Which is fine enough for my purposes but this is ok only if the notebook was just created or you know that there aren't any TaggingRules.

---

The problem of course appears when you don't know and you don't want to overwrite them with {}. Options / CurrentValue (Absolute or not) will only tell you what is there with already included $FrontEnd values, so to check what is explicitly written you have to call NotebookGet, and it doesn't scale well as notebook size grows.

So to check you can run:

MemberQ[TaggingRules -> _] @ Rest @ NotebookGet[]

---

p.s. there is an undocumented syntax for CurrentValue so that by using:

CurrentValue[$FrontEndSession, {TaggingRules, "test"}, 2]

we say "give me TaggingRules 'test' value and if it doesn't exist, set it to 2 and return 2. Very convenient. But

CurrentValue[EvaluationNotebook[], TaggingRules, {}]

doesn't work in this case :)