[c++] Fix dump_model() information for root node

[neNasko1]

This PR corrects the output of dump_model() and other dump-related functions like trees_to_dataframe(). There are 2 fixes implemented:

1. The current Tree::Split implementation incorrectly saves the old leaf output value in the internal_value_ array when called on the root node. This in turn makes inspecting the whole training process from python incomplete.

Before:

(Pdb) booster_.trees_to_dataframe()
     tree_index  node_depth node_index left_child right_child parent_index  ... decision_type  missing_direction missing_type     value weight count
0             0           1       0-S0       0-S1        0-S2         None  ...            ==              right         None  0.000000      0   200
1             0           2       0-S1       0-S5        0-S4         0-S0  ...            <=               left         None  0.106573    113   113
2             0           3       0-S5       0-L0        0-L6         0-S1  ...            ==              right         None  0.082122     56    56
3             0           4       0-L0       None        None         0-S5  ...          None               None         None  0.064612     26    26
4             0           4       0-L6       None        None         0-S5  ...          None               None         None  0.097297     30    30

After:

(Pdb) booster_.trees_to_dataframe().head()
   tree_index  node_depth node_index left_child right_child parent_index  ... decision_type  missing_direction missing_type     value weight count
0           0           1       0-S0       0-S1        0-S2         None  ...            ==              right         None  0.081757    200   200
1           0           2       0-S1       0-S5        0-S4         0-S0  ...            <=               left         None  0.106573    113   113
2           0           3       0-S5       0-L0        0-L6         0-S1  ...            ==              right         None  0.082122     56    56
3           0           4       0-L0       None        None         0-S5  ...          None               None         None  0.064612     26    26
4           0           4       0-L6       None        None         0-S5  ...          None               None         None  0.097297     30    30

2. Stump has no leaf_count inside dump_model() output #5962

[neNasko1]

Currently the CI is not passing as #6574 is blocking.

[neNasko1]

I am open to ideas of ways to test related functionalities.

Tests should now be sufficient for the change.

[neNasko1]

@jameslamb
Could you take a look at the PR, now that the CI is passing?

[jameslamb]

@shiyu1994 or @guolinke could you help with a review of this?

I'm not sure if this will correctly handle these cases:

• custom init_score provided (via Dataset)
• boost_from_average=False passed

@neNasko1 could you also look at #5962 and let us know if you think this change would fix the issue @thatlittleboy reported there?

[neNasko1]

Thank you for taking the time to look into the PR and linking a relevant issue.

I'm not sure if this will correctly handle these cases:

• custom init_score provided (via Dataset)
• boost_from_average=False passed

I think those cases are handled as the results are consistent with what leaf values report, I also remade the test to boost from average.

@neNasko1 could you also look at #5962 and let us know if you think this change would fix the issue @thatlittleboy reported there?

I took the liberty to merge @thatlittleboy's WIP code into mine, additionally fixing the issues that they reported. I will also change the description of the PR to reflect both of the fixes.

[StrikerRUS]

LGTM!

But I'll keep following the discussion about Dask Ranker test (#6569 (comment)).

[neNasko1]

@jameslamb can you submit a final review on the change, so that we can merge it?

Sorry for the caused inconvenience!

[jameslamb]

I will look when I can. I have spent most of my limited open source time in the last few weeks investigating and fixing multiple difficult, time-sensitive CI issues in this project, and there is yet another one that is still not done and a primary focus for me right now (#6651).

If @StrikerRUS has time to re-review the commits and comments you've pushed since his approval, and if he approves, then my review can be dismissed and this can be merged without another review from me. Otherwise, you will have to be patient a bit longer.

[jameslamb]

/gha run r-valgrind

Workflow R valgrind tests has been triggered! 🚀
https://github.com/microsoft/LightGBM/actions/runs/11226953247

Status: success ✔️.

[jameslamb]

Thanks. I've left some minor suggestions for your consideration, around making the tests stricter and easier to understand.

I've also triggered valgrind checks on this branch, to ensure no new memory-management issues have been introduced by this PR.

Unfortunately, this still needs a bit more investigation before I'm confident in it... I was finally able to investigate your comments in #6569 (comment), and found that that Dask test checking the trees_to_dataframe() output in the presence of init_score really was testing that the init_score wasn't ignored. I am going to try right now to figure out why that was, and if it has implications for this PR.

---

By the way, most of the commits you've pushed here are not tied to your GitHub account.

Doesn't really matter in this repo because if this is merged we'll squash everything into one commit, and that'll be correctly tied to your account. But just making you aware of it as it might cause problems for you in other GitHub-based projects. You can fix that for future commits like this:

git config --global user.email "${EMAIL}"

replacing ${EMAIL} with an email address tied to your GitHub account.

[neNasko1]

I will look when I can. I have spent most of my limited open source time in the last few weeks investigating and fixing multiple difficult, time-sensitive CI issues in this project, and there is yet another one that is still not done and a primary focus for me right now (#6651).

Sorry for making it seem like there is some rush around this. I understand that this project is run mainly by volunteers and I do not want to harass any of the maintainers.

Thanks. I've left some minor suggestions for your consideration, around making the tests stricter and easier to understand.
I've also triggered valgrind checks on this branch, to ensure no new memory-management issues have been introduced by this PR.

Thanks for the suggestions, all of them are reasonable and are now merged. I grouped them into one commit as I wanted to test if everything is okay.

Again, thanks to everyone for the time spent!

[neNasko1]

@jameslamb

With init_score not provided, the root node's value is a non-0 value, and the test reliably and consistently fail.

Is this related to boost_from_average?

• When there is a init_score then boost_from_average does not occur.
• After training the first trees(iteration=1) we then add the average to them.

I retract my comment of the test being a no-op previously, however after this fix there is a need for a change in this test.

[jameslamb]

Thanks, changes look great. For your other questions, let's please stay in the thread you're quoting, so the conversation can all be grouped together. I've responded there: #6569 (comment)

[jameslamb]

This is looking great! Just some small suggestions on the new Dask test, and then I think we can merge this. Thanks for all your hard work, I'm glad we'll be able to get this fix in.

[jameslamb]

This is looking good, thanks very much for the help!

[neNasko1]

This is looking good, thanks very much for the help!

Thanks for the all the time spent! Glad to see it merged 🚀