Implement $list$explode() (#1139)

NEWS.md

@@ -13,13 +13,10 @@
   - for the Expr method only, the default value for `with_replacement` is now
     `FALSE` (it was already the case for the DataFrame method).
 
-
-
-
-
 ### New features
 
 - New method `$has_nulls()` (#1133).
+- New method `$list$explode()` (#1139).
 
 ## Polars R Package 0.17.0
 

R/expr__list.R

@@ -599,3 +599,14 @@ ExprList_set_symmetric_difference = function(other) {
   .pr$Expr$list_set_operation(self, other, "symmetric_difference") |>
     unwrap("in $list$set_symmetric_difference():")
 }
+
+#' Returns a column with a separate row for every list element
+#'
+#' @inherit ExprList_set_union return
+#'
+#' @examples
+#' df = pl$DataFrame(a = list(c(1, 2, 3), c(4, 5, 6)))
+#' df$select(pl$col("a")$list$explode())
+ExprList_explode = function() {
+  .pr$Expr$explode(self)
+}

tests/testthat/test-expr_list.R

@@ -614,3 +614,15 @@ test_that("$list$set_*() casts to common supertype", {
     list(a = list(c("1.0", "2.0", "a", "b"), character(0)))
   )
 })
+
+test_that("$list$explode() works", {
+  df = pl$DataFrame(a = list(c(1, 2, 3), c(4, 5, 6)))
+  expect_identical(
+    df$select(pl$col("a")$list$explode())$to_list(),
+    list(a = c(1, 2, 3, 4, 5, 6))
+  )
+  expect_error(
+    df$with_columns(pl$col("a")$list$explode()),
+    "lengths don't match"
+  )
+})