Implement .pr$DataFrame$drop_all_in_place() (#504)

NEWS.md

@@ -15,6 +15,10 @@
 - New methods `$write_json()` and `$write_ndjson()` for DataFrame (#502).
 - Removed argument `name` in `pl$date_range()`, which was deprecated for a while
   (#503).
+- New private method `.pr$DataFrame$drop_all_in_place(df)` to drop `DataFrame` in-place,
+ to release memory without invoking gc(). However, if there are other strong references to any of
+ the underlying Series or arrow arrays, that memory will specifically not be released. This method
+ is aimed for r-polars extensions, and will be kept stable as much as possible (#504).
 - New functions `pl$min_horizontal()`, `pl$max_horizontal()`, `pl$sum_horizontal()`,
   `pl$all_horizontal()`, `pl$any_horizontal()` (#508).
 

R/extendr-wrappers.R

@@ -135,6 +135,8 @@ DataFrame$default <- function() .Call(wrap__DataFrame__default)
 
 DataFrame$lazy <- function() .Call(wrap__DataFrame__lazy, self)
 
+DataFrame$drop_all_in_place <- function() invisible(.Call(wrap__DataFrame__drop_all_in_place, self))
+
 DataFrame$new_with_capacity <- function(capacity) .Call(wrap__DataFrame__new_with_capacity, capacity)
 
 DataFrame$set_column_from_robj <- function(robj, name) .Call(wrap__DataFrame__set_column_from_robj, self, robj, name)

src/rust/src/rdataframe/mod.rs

@@ -124,6 +124,11 @@ impl DataFrame {
         LazyFrame(self.0.clone().lazy())
     }
 
+    //internal use only
+    pub fn drop_all_in_place(&mut self) {
+        *self = Self::new_with_capacity(0);
+    }
+
     //internal use
     pub fn new_with_capacity(capacity: i32) -> Self {
         let empty_series: Vec<pl::Series> = Vec::with_capacity(capacity as usize);

tests/testthat/test-dataframe.R

@@ -1206,3 +1206,19 @@ test_that("transpose", {
     df_expected
   )
 })
+
+test_that("drop_all_in_place", {
+
+  # this test verifies internal function in_place drop all Series in DataFrame
+  # will not affect a fully cloned DataFrame df_clone
+
+  df_copy = df = polars::pl$DataFrame(mtcars)
+  df_clone = df$clone()
+  s = df$get_column("cyl")
+  .pr$DataFrame$drop_all_in_place(df)
+  expect_identical(df$shape, c(0,0))
+  expect_identical(df_copy$shape, c(0,0))
+  expect_identical(df_clone$shape, c(32,11))
+  expect_identical(s$len(), 32)
+})
+