feat: keep_results can be character vector of IDs

This can be useful when wanting access to an object that is
not an output node of the graph, i.e. we don't have to add
a `PipeOpNOP` (or keep all results) to achieve this.

NEWS.md

@@ -1,5 +1,8 @@
 # mlr3pipelines 0.5.0-9000
 
+* Feature: The `Graph`'s `keep_results` can now also be a character vector
+containing the IDs of the `PipeOp`s whose results are being stored.
+
 # mlr3pipelines 0.5.0-1
 
 * Bugfix: `PipeOpTuneThreshold` was not overloading the correct `.train` and `.predict` functions.

R/Graph.R

@@ -59,8 +59,9 @@
 #' * `phash` :: `character(1)` \cr
 #'   Stores a checksum calculated on the [`Graph`] configuration, which includes all [`PipeOp`] hashes
 #'   *except* their `$param_set$values`, and a hash of `$edges`.
-#' * `keep_results` :: `logical(1)` \cr
+#' * `keep_results` :: `logical(1)` or `character()` \cr
 #'   Whether to store intermediate results in the [`PipeOp`]'s `$.result` slot, mostly for debugging purposes. Default `FALSE`.
+#'   Can also be a character vector of IDs, in which case only the results of the selected `PipeOp`s are stored.
 #' * `man` :: `character(1)`\cr
 #'   Identifying string of the help page that shows with `help()`.
 #'
@@ -642,7 +643,7 @@ graph_reduce = function(self, input, fun, single_input) {
     lg$debug("Running PipeOp '%s$%s()'", id, fun, pipeop = op, input = input)
 
     output = op[[fun]](input)
-    if (self$keep_results) {
+    if (isTRUE(self$keep_results) || op$id %in% self$keep_results) {
       op$.result = output
     }
 

R/PipeOp.R

@@ -130,7 +130,7 @@
 #'   [`PipeOp`]'s functionality may change depending on more than these values, it should inherit the `$hash` active
 #'   binding and calculate the hash as `digest(list(super$hash, <OTHER THINGS>), algo = "xxhash64")`.
 #' * `.result` :: `list` \cr
-#'   If the [`Graph`]'s `$keep_results` flag is set to `TRUE`, then the intermediate Results of `$train()` and `$predict()`
+#'   If the [`Graph`]'s `$keep_results` flag is set to `TRUE` or contains the ID of this `PipeOp`, then the intermediate Results of `$train()` and `$predict()`
 #'   are saved to this slot, exactly as they are returned by these functions. This is mainly for debugging purposes
 #'   and done, if requested, by the [`Graph`] backend itself; it should *not* be done explicitly by `private$.train()` or `private$.predict()`.
 #' * `man` :: `character(1)`\cr

tests/testthat/test_Graph.R

@@ -501,3 +501,14 @@ test_that("Same output into multiple channels does not cause a bug", {
   expect_true(res$po3.output1 == 2)
   expect_true(res$po4.output1 == 2)
 })
+
+test_that("keep_results can be a character vector", {
+  graph = po("pca") %>>% po("ica")
+
+  graph$keep_results = "pca"
+
+  graph$train(tsk("iris"))
+
+  expect_true(is.null(graph$pipeops$ica$.result))
+  expect_class(graph$pipeops$pca$.result[[1L]], "Task")
+})