fix: Fix melt panic when there are no value vars (#14057)

crates/polars-core/src/frame/explode.rs

@@ -259,13 +259,25 @@ impl DataFrame {
         let id_vars = args.id_vars;
         let mut value_vars = args.value_vars;
 
-        let value_name = args.value_name.as_deref().unwrap_or("value");
         let variable_name = args.variable_name.as_deref().unwrap_or("variable");
+        let value_name = args.value_name.as_deref().unwrap_or("value");
 
         let len = self.height();
 
         // if value vars is empty we take all columns that are not in id_vars.
         if value_vars.is_empty() {
+            // return empty frame if there are no columns available to use as value vars
+            if id_vars.len() == self.width() {
+                let variable_col = Series::new_empty(variable_name, &DataType::String);
+                let value_col = Series::new_empty(variable_name, &DataType::Null);
+
+                let mut out = self.select(id_vars).unwrap().clear().columns;
+                out.push(variable_col);
+                out.push(value_col);
+
+                return Ok(DataFrame::new_no_checks(out));
+            }
+
             let id_vars_set = PlHashSet::from_iter(id_vars.iter().map(|s| s.as_str()));
             value_vars = self
                 .get_columns()
@@ -325,7 +337,7 @@ impl DataFrame {
 
         let variable_col = variable_col.as_box();
         // Safety
-        // The give dtype is correct
+        // The given dtype is correct
         let variables = unsafe {
             Series::from_chunks_and_dtype_unchecked(
                 variable_name,

crates/polars-plan/src/logical_plan/builder_functions.rs

@@ -35,29 +35,23 @@ pub(super) fn det_melt_schema(args: &MeltArgs, input_schema: &Schema) -> SchemaR
     new_schema.with_column(variable_name, DataType::String);
 
     // We need to determine the supertype of all value columns.
-    let mut st = None;
+    let mut supertype = DataType::Null;
 
     // take all columns that are not in `id_vars` as `value_var`
     if args.value_vars.is_empty() {
         let id_vars = PlHashSet::from_iter(&args.id_vars);
         for (name, dtype) in input_schema.iter() {
             if !id_vars.contains(name) {
-                match &st {
-                    None => st = Some(dtype.clone()),
-                    Some(st_) => st = Some(try_get_supertype(st_, dtype).unwrap()),
-                }
+                supertype = try_get_supertype(&supertype, dtype).unwrap();
             }
         }
     } else {
         for name in &args.value_vars {
             let dtype = input_schema.get(name).unwrap();
-            match &st {
-                None => st = Some(dtype.clone()),
-                Some(st_) => st = Some(try_get_supertype(st_, dtype).unwrap()),
-            }
+            supertype = try_get_supertype(&supertype, dtype).unwrap();
         }
     }
-    new_schema.with_column(value_name, st.unwrap());
+    new_schema.with_column(value_name, supertype);
     Arc::new(new_schema)
 }
 

py-polars/tests/unit/operations/test_melt.py

@@ -69,3 +69,15 @@ def test_melt_projection_pd_7747() -> None:
         }
     )
     assert_frame_equal(result, expected)
+
+
+# https://github.com/pola-rs/polars/issues/10075
+def test_melt_no_value_vars() -> None:
+    lf = pl.LazyFrame({"a": [1, 2, 3]})
+
+    result = lf.melt("a")
+
+    expected = pl.LazyFrame(
+        schema={"a": pl.Int64, "variable": pl.String, "value": pl.Null}
+    )
+    assert_frame_equal(result, expected)