Strange behavior of Out[]

Question

Let's create two notebooks external.nb and primary.nb. Then call the external.nb notebook from the primary.nb and investigate values of Out[ ]. The context of the external notebook is not important, however for the demonstration of the strange behavior it should contain number of cells. My external.nb notebook contains 4 cells:

a=1; 

---

b=1

---

c=1

---

d

The primary notebook contains the single cell which calls the external notebook from the primary. If we put both notebooks in the same directory, the cell is

NotebookEvaluate @ FileNameJoin[
  { NotebookDirectory[EvaluationNotebook[]], "external.nb"}
]

Open the primary.nb and evaluate the cell which calls the evaluation of the external.nb.

The output yields

Out[1] = d

So it returns output of the last cell of the called notebook.

Now refer to input %4 of primary.nb. I would expect an empty output, since at the moment there is no %4 out in the primary notebook. However the result is the same d.

Asking

?? Out[]

we see the primary.nb knows all Out[] values of the external.nb notebook. Bug or feature?

Answer 1

I am guessing that this has to do with the following note in the NotebookEvaluate documentation:

The cells of the notebook are evaluated in a dialog subsession.

Here is an example session demonstrating similar behaviour:

See Dialog if you are not familiar with it.

Within the dialog, $Line keeps getting incremented as usual, and Out values are set. But once the dialog finishes, $Line is reset to its original value.

This is documented:

Dialog automatically localizes the values of $Line, $MessageList, and $Epilog.

Side note: A dialog is something you would normally encounter not when explicitly entering it, but when interrupting evaluation or when debugging (say, you set the debugger to break on a message/assert). Try e.g. Do[Pause[1], {i, 30}], then interrupt using Alt-, (Option-Command-. on Mac). Now you can evaluate i to check its value, or examine the kernel state in other ways. Exit the dialog using Return[]. It is natural that one would not want these evaluations to influence $Line once the dialog has finished.