fix(rust, python): empty concat utf8 (#5768)

polars/polars-core/src/functions.rs

@@ -143,7 +143,7 @@ pub fn concat_str(s: &[Series], delimiter: &str) -> PolarsResult<Utf8Chunked> {
             "expected multiple series in concat_str function".into(),
         ));
     }
-    if s[0].is_empty() {
+    if s.iter().any(|s| s.is_empty()) {
         return Ok(Utf8Chunked::full_null(s[0].name(), 0));
     }
 
@@ -165,7 +165,7 @@ pub fn concat_str(s: &[Series], delimiter: &str) -> PolarsResult<Utf8Chunked> {
 
     if !s.iter().all(|s| s.len() == 1 || s.len() == len) {
         return Err(PolarsError::ComputeError(
-            "all series in concat_str function should have equal length or unit length".into(),
+            "All series in concat_str function should have equal length or unit length".into(),
         ));
     }
     let mut iters = cas

polars/polars-ops/src/frame.rs → polars/polars-ops/src/frame/mod.rs

@@ -1,3 +1,5 @@
+#[cfg(feature = "pivot")]
+pub mod pivot;
 #[cfg(feature = "to_dummies")]
 use polars_core::export::rayon::prelude::*;
 use polars_core::prelude::*;

polars/polars-ops/src/lib.rs

@@ -1,7 +1,7 @@
 #![cfg_attr(docsrs, feature(doc_cfg))]
 mod chunked_array;
-mod frame;
 #[cfg(feature = "pivot")]
-pub mod pivot;
+pub use frame::pivot;
+mod frame;
 pub mod prelude;
 mod series;

py-polars/tests/unit/test_empty.py

@@ -0,0 +1,10 @@
+import polars as pl
+
+
+def test_empty_str_concat_lit() -> None:
+    df = pl.DataFrame({"a": [], "b": []}, columns=[("a", pl.Utf8), ("b", pl.Utf8)])
+    assert df.with_column(pl.lit("asd") + pl.col("a")).schema == {
+        "a": pl.Utf8,
+        "b": pl.Utf8,
+        "literal": pl.Utf8,
+    }