[R-package] only warn about early stopping and DART boosting being incompatible if early stopping was requested (#6619)

R-package/R/lgb.cv.R

@@ -295,7 +295,9 @@ lgb.cv <- function(params = list()
 
   # Cannot use early stopping with 'dart' boosting
   if (using_dart) {
-    warning("Early stopping is not available in 'dart' mode.")
+    if (using_early_stopping) {
+      warning("Early stopping is not available in 'dart' mode.")
+    }
     using_early_stopping <- FALSE
 
     # Remove the cb_early_stop() function if it was passed in to callbacks

R-package/R/lgb.train.R

@@ -258,7 +258,9 @@ lgb.train <- function(params = list(),
 
   # Cannot use early stopping with 'dart' boosting
   if (using_dart) {
-    warning("Early stopping is not available in 'dart' mode.")
+    if (using_early_stopping) {
+      warning("Early stopping is not available in 'dart' mode.")
+    }
     using_early_stopping <- FALSE
 
     # Remove the cb_early_stop() function if it was passed in to callbacks

R-package/tests/testthat/test_parameters.R

@@ -91,7 +91,7 @@ test_that(".PARAMETER_ALIASES() uses the internal session cache", {
   expect_false(exists(cache_key, where = .lgb_session_cache_env))
 })
 
-test_that("training should warn if you use 'dart' boosting, specified with 'boosting' or aliases", {
+test_that("training should warn if you use 'dart' boosting with early stopping", {
   for (boosting_param in .PARAMETER_ALIASES()[["boosting"]]) {
     params <- list(
         num_leaves = 5L
@@ -101,14 +101,69 @@ test_that("training should warn if you use 'dart' boosting, specified with 'boos
         , num_threads = .LGB_MAX_THREADS
     )
     params[[boosting_param]] <- "dart"
+
+    # warning: early stopping requested
     expect_warning({
       result <- lightgbm(
         data = train$data
         , label = train$label
         , params = params
-        , nrounds = 5L
-        , verbose = -1L
+        , nrounds = 2L
+        , verbose = .LGB_VERBOSITY
+        , early_stopping_rounds = 1L
       )
     }, regexp = "Early stopping is not available in 'dart' mode")
+
+    # no warning: early stopping not requested
+    expect_silent({
+      result <- lightgbm(
+        data = train$data
+        , label = train$label
+        , params = params
+        , nrounds = 2L
+        , verbose = .LGB_VERBOSITY
+        , early_stopping_rounds = NULL
+      )
+    })
+  }
+})
+
+test_that("lgb.cv() should warn if you use 'dart' boosting with early stopping", {
+  for (boosting_param in .PARAMETER_ALIASES()[["boosting"]]) {
+    params <- list(
+      num_leaves = 5L
+      , objective = "binary"
+      , metric = "binary_error"
+      , num_threads = .LGB_MAX_THREADS
+    )
+    params[[boosting_param]] <- "dart"
+
+    # warning: early stopping requested
+    expect_warning({
+      result <- lgb.cv(
+        data = lgb.Dataset(
+          data  = train$data
+          , label = train$label
+        )
+        , params = params
+        , nrounds = 2L
+        , verbose = .LGB_VERBOSITY
+        , early_stopping_rounds = 1L
+      )
+    }, regexp = "Early stopping is not available in 'dart' mode")
+
+    # no warning: early stopping not requested
+    expect_silent({
+      result <- lgb.cv(
+        data = lgb.Dataset(
+          data  = train$data
+          , label = train$label
+        )
+        , params = params
+        , nrounds = 2L
+        , verbose = .LGB_VERBOSITY
+        , early_stopping_rounds = NULL
+      )
+    })
   }
 })