Output of code generated by CellPrint goes to the end of notebook

Question

Bug introduced in 3.0 and persisting through 13.0.1. Wolfram Cloud is not influenced.

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This is an issue raised up when I use the function importCode, the following is a simplified sample reproducing the issue. First execute the following line to create 2 cells:

Scan[CellPrint@Cell[#, "Input"] &, {"1\n2", "3"}]

Then execute the first generated cell, you'll see:

As we can see, the output 2 goes to the end of notebook unexpectedly. (If you go on executing the cell containing 3, the 2 in output cell is eaten. This is expected, though. ) Is this a bug or by design? Any work-around? (Because of this, the function importCode isn't working well on code block with multiple outputs for the moment. )

Answer 1

I don't have full insight so I am encouraging FE team to answer or update this one. Currently I can only more or less explain what is going on but not why/what was the original idea.

The rule of thumb is: CellPrint should be used to generate output-ish cells. Use NotebookWrite for a full control.

Confirmation of the rule are those comments by John Fultz

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Explanation

CellsPrint adds CellAutoOverwrite and GeneratedCell options to what it creates so we can already see that those options together with Input style will be problematic.

FE actions when kernel returns an evaluated cell's expression are (based on rough experiments):

1. Delete all subsequent cells which are CellAutoOverwrite but not Evaluatable.
2. Insert Cell
3. Move selection after subsequent GeneratedCells. (Notice the Output cells are GeneratedCells by default)

Sounds reasonable. Maybe you could think about something simpler. Maybe the 3rd rule should exclude Evaluatable again, I don't know, but keep in mind that it should be 30y backward compatible and it has to handle issues like the bug mentioned by JF in the linked comment.

Having those rules explains OP problem, 3 is not good enough to be deleted (has Evaluatable) but selection will move past it (GeneratedCell). It looks weird but is solved with NotebookWrite or if we only print output-ish (non Evaluatable) cells with CellPrint.

Alexey problem is explained as well: Text cell will be deleted because it has CellAutoOverwrite and does not have Evaluatable settings.

Answer 2

I reproduce this behavior with versions 5.2, 8.0.4, 12.3.1 and 13.0.1 on Win10 x64.

I would count this as a long-standing FrontEnd bug. An extended example:

CellPrint /@ {
   Cell["\"First output from Cell#1\"\n\"Second output from Cell#1.  Whoops! How did I get here ???\"", "Input", CellLabel -> "Cell#1"],
   Cell["I'll disappear after evaluation of the previous cell!", "Text",  CellLabel -> "Cell#2"],
   Cell["3", "Input", CellLabel -> "Cell#3"]};

After evaluation of the first printed input cell with label "Cell#1", the "Text" cell with label "Cell#2" disappears. And the second output from "Cell#1" is unexpectedly printed after "Cell#3"!

As a workaround I can suggest using NotebookWrite instead of CellPrint:

NotebookWrite[EvaluationNotebook[], #] & /@ {Cell["1\n2", "Input"], 
   Cell["I won't disappear after evaluation of the previous cell!", "Text"], 
   Cell["3", "Input"]};

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Further observations

It's also worth noting that the documentation for CellPrint contradicts its observed behavior. In particulat, it states (in the very first section under the function name):

CellPrint[expr]
inserts expr as a complete cell in the current notebook just below the cell being evaluated.

However, by default CellPrint[expr] inserts expr below the last cell generated by the cell being evaluated (and in this sense behaves exactly like Print):

"Main output"
Print["First Print"]
CellPrint[Cell["First CellPrint", "Input"]]
Print["Second Print"]
CellPrint[TextCell["Second CellPrint", "Text"]]

Something similar to the documented behavior happens when we print cells with GeneratedCell -> False option. But as one can see, in this case CellPrint[expr] inserts expr below the last cell with GeneratedCell -> True generated by the cell being evaluated:

"Main output"
Print["First Print"]
CellPrint[TextCell["First CellPrint", "Input", GeneratedCell -> False]]
Print["Second Print"]
CellPrint[TextCell["Second CellPrint", "Text", GeneratedCell -> False]]

Hence we also have a Documentation bug.

To summarize the above observations:

CellPrint[expr] inserts expr below the cell being evaluated after the last cell with GeneratedCell -> True above the next cell with GeneratedCell -> False, even if the latter was generated during current evaluation.

Or shorter (official documentation style):

CellPrint[expr] inserts expr just above the next cell with GeneratedCell -> False, or at the end of the evaluation notebook.

However, in order to fully understand the results of CellPrint, one must take into account the general behavior of the FrontEnd when it receives the evaluation result (described in the "Explanation" section of the answer by Kuba). In the first place, one should pay attention to the fact that as the first step the FrontEnd deletes all subsequent cells which have CellAutoOverwrite -> True but not Evaluatable -> True.