lgb.load(filename = NULL, model_str = NULL)

data(agaricus.train, package = "lightgbm")
train <- agaricus.train
dtrain <- lgb.Dataset(train$data, label = train$label)
data(agaricus.test, package = "lightgbm")
test <- agaricus.test
dtest <- lgb.Dataset.create.valid(dtrain, test$data, label = test$label)
params <- list(objective = "regression", metric = "l2")
valids <- list(test = dtest)
model <- lgb.train(
  params = params
  , data = dtrain
  , nrounds = 5L
  , valids = valids
  , min_data = 1L
  , learning_rate = 1.0
  , early_stopping_rounds = 3L
)
model_file <- tempfile(fileext = ".txt")
lgb.save(model, model_file)
load_booster <- lgb.load(filename = model_file)
model_string <- model$save_model_to_string(NULL) # saves best iteration
load_booster_from_str <- lgb.load(model_str = model_string)