From 4a3957682e87dc2af63b4197e9376ec557e6f36c Mon Sep 17 00:00:00 2001
From: Ritchie Vink <ritchie46@gmail.com>
Date: Sun, 22 May 2022 09:12:37 +0200
Subject: [PATCH] explode series after slide fast path (#3467)

---
 polars/polars-core/src/frame/explode.rs | 8 +++++++-
 py-polars/src/file.rs                   | 2 +-
 py-polars/tests/test_explode.py         | 5 +++++
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/polars/polars-core/src/frame/explode.rs b/polars/polars-core/src/frame/explode.rs
index 4af4ba3fa8a3..4eeca11d43af 100644
--- a/polars/polars-core/src/frame/explode.rs
+++ b/polars/polars-core/src/frame/explode.rs
@@ -52,7 +52,13 @@ impl DataFrame {
                 if !ca.can_fast_explode() {
                     let (_, offsets) = get_exploded(col)?;
                     if offsets.is_empty() {
-                        return Ok(self.slice(0, 0));
+                        let mut out = self.slice(0, 0);
+                        // still explode the columns to get the inner dtype
+                        for col in &columns {
+                            out.try_apply(col.name(), |s| s.explode())?;
+                        }
+
+                        return Ok(out);
                     }
 
                     let mut mask = MutableBitmap::from_len_set(offsets.len() - 1);
diff --git a/py-polars/src/file.rs b/py-polars/src/file.rs
index 2d2858ba6a55..7455cf077034 100644
--- a/py-polars/src/file.rs
+++ b/py-polars/src/file.rs
@@ -212,7 +212,7 @@ pub fn get_either_file(py_f: PyObject, truncate: bool) -> PyResult<EitherRustPyt
         } else {
             match File::open(str_slice) {
                 Ok(file) => BufReader::new(file),
-                Err(e) => {
+                Err(_e) => {
                     return Err(PyErr::new::<PyFileNotFoundError, _>(format!(
                         "No such file or directory: {}",
                         str_slice
diff --git a/py-polars/tests/test_explode.py b/py-polars/tests/test_explode.py
index c6d0a789304e..a97b05e5da86 100644
--- a/py-polars/tests/test_explode.py
+++ b/py-polars/tests/test_explode.py
@@ -6,3 +6,8 @@
 def test_explode_empty_df_3402() -> None:
     df = pl.DataFrame({"a": pa.array([], type=pa.large_list(pa.int32()))})
     assert df.explode("a").dtypes == [pl.Int32]
+
+
+def test_explode_empty_df_3460() -> None:
+    df = pl.DataFrame({"a": pa.array([[]], type=pa.large_list(pa.int32()))})
+    assert df.explode("a").dtypes == [pl.Int32]