feat: rename DataFrame.melt to DataFrame.unpivot, and rename its args:

id_vars -> index, value_vars -> on

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

@@ -20,15 +20,15 @@ fn get_exploded(series: &Series) -> PolarsResult<(Series, OffsetsBuffer<i64>)> {
     }
 }
 
-/// Arguments for `[DataFrame::melt]` function
+/// Arguments for `[DataFrame::unpivot]` function
 #[derive(Clone, Default, Debug, PartialEq, Eq, Hash)]
 #[cfg_attr(feature = "serde-lazy", derive(Serialize, Deserialize))]
-pub struct MeltArgs {
-    pub id_vars: Vec<SmartString>,
-    pub value_vars: Vec<SmartString>,
+pub struct UnpivotArgs {
+    pub index: Vec<SmartString>,
+    pub on: Vec<SmartString>,
     pub variable_name: Option<SmartString>,
     pub value_name: Option<SmartString>,
-    /// Whether the melt may be done
+    /// Whether the unpivot may be done
     /// in the streaming engine
     /// This will not have a stable ordering
     pub streamable: bool,
@@ -189,10 +189,10 @@ impl DataFrame {
     ///
     /// # Arguments
     ///
-    /// * `id_vars` - String slice that represent the columns to use as id variables.
-    /// * `value_vars` - String slice that represent the columns to use as value variables.
+    /// * `index` - String slice that represent the columns to use as id variables.
+    /// * `on` - String slice that represent the columns to use as value variables.
     ///
-    /// If `value_vars` is empty all columns that are not in `id_vars` will be used.
+    /// If `on` is empty all columns that are not in `index` will be used.
     ///
     /// ```ignore
     /// # use polars_core::prelude::*;
@@ -202,9 +202,9 @@ impl DataFrame {
     ///              "D" => &[2, 4, 6]
     ///     )?;
     ///
-    /// let melted = df.melt(&["A", "B"], &["C", "D"])?;
+    /// let unpivoted = df.unpivot(&["A", "B"], &["C", "D"])?;
     /// println!("{:?}", df);
-    /// println!("{:?}", melted);
+    /// println!("{:?}", unpivoted);
     /// # Ok::<(), PolarsError>(())
     /// ```
     /// Outputs:
@@ -239,51 +239,51 @@ impl DataFrame {
     ///  | "a" | 5   | "D"      | 6     |
     ///  +-----+-----+----------+-------+
     /// ```
-    pub fn melt<I, J>(&self, id_vars: I, value_vars: J) -> PolarsResult<Self>
+    pub fn unpivot<I, J>(&self, index: I, on: J) -> PolarsResult<Self>
     where
         I: IntoVec<SmartString>,
         J: IntoVec<SmartString>,
     {
-        let id_vars = id_vars.into_vec();
-        let value_vars = value_vars.into_vec();
-        self.melt2(MeltArgs {
-            id_vars,
-            value_vars,
+        let id_vars = index.into_vec();
+        let value_vars = on.into_vec();
+        self.unpivot2(UnpivotArgs {
+            index: id_vars,
+            on: value_vars,
             ..Default::default()
         })
     }
 
-    /// Similar to melt, but without generics. This may be easier if you want to pass
-    /// an empty `id_vars` or empty `value_vars`.
-    pub fn melt2(&self, args: MeltArgs) -> PolarsResult<Self> {
-        let id_vars = args.id_vars;
-        let mut value_vars = args.value_vars;
+    /// Similar to unpivot, but without generics. This may be easier if you want to pass
+    /// an empty `index` or empty `on`.
+    pub fn unpivot2(&self, args: UnpivotArgs) -> PolarsResult<Self> {
+        let index = args.index;
+        let mut on = args.on;
 
         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() {
+        if on.is_empty() {
             // return empty frame if there are no columns available to use as value vars
-            if id_vars.len() == self.width() {
+            if index.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;
+                let mut out = self.select(index).unwrap().clear().columns;
                 out.push(variable_col);
                 out.push(value_col);
 
                 return Ok(unsafe { DataFrame::new_no_checks(out) });
             }
 
-            let id_vars_set = PlHashSet::from_iter(id_vars.iter().map(|s| s.as_str()));
-            value_vars = self
+            let index_set = PlHashSet::from_iter(index.iter().map(|s| s.as_str()));
+            on = self
                 .get_columns()
                 .iter()
                 .filter_map(|s| {
-                    if id_vars_set.contains(s.name()) {
+                    if index_set.contains(s.name()) {
                         None
                     } else {
                         Some(s.name().into())
@@ -294,7 +294,7 @@ impl DataFrame {
 
         // values will all be placed in single column, so we must find their supertype
         let schema = self.schema();
-        let mut iter = value_vars.iter().map(|v| {
+        let mut iter = on.iter().map(|v| {
             schema
                 .get(v)
                 .ok_or_else(|| polars_err!(ColumnNotFound: "{}", v))
@@ -304,31 +304,30 @@ impl DataFrame {
             st = try_get_supertype(&st, dt?)?;
         }
 
-        // The column name of the variable that is melted
-        let mut variable_col =
-            MutableBinaryViewArray::<str>::with_capacity(len * value_vars.len() + 1);
+        // The column name of the variable that is unpivoted
+        let mut variable_col = MutableBinaryViewArray::<str>::with_capacity(len * on.len() + 1);
         // prepare ids
-        let ids_ = self.select_with_schema_unchecked(id_vars, &schema)?;
+        let ids_ = self.select_with_schema_unchecked(index, &schema)?;
         let mut ids = ids_.clone();
         if ids.width() > 0 {
-            for _ in 0..value_vars.len() - 1 {
+            for _ in 0..on.len() - 1 {
                 ids.vstack_mut_unchecked(&ids_)
             }
         }
         ids.as_single_chunk_par();
         drop(ids_);
 
-        let mut values = Vec::with_capacity(value_vars.len());
+        let mut values = Vec::with_capacity(on.len());
 
-        for value_column_name in &value_vars {
+        for value_column_name in &on {
             variable_col.extend_constant(len, Some(value_column_name.as_str()));
             // ensure we go via the schema so we are O(1)
             // self.column() is linear
-            // together with this loop that would make it O^2 over value_vars
+            // together with this loop that would make it O^2 over `on`
             let (pos, _name, _dtype) = schema.try_get_full(value_column_name)?;
             let col = &self.columns[pos];
             let value_col = col.cast(&st).map_err(
-                |_| polars_err!(InvalidOperation: "'melt/unpivot' not supported for dtype: {}", col.dtype()),
+                |_| polars_err!(InvalidOperation: "'unpivot' not supported for dtype: {}", col.dtype()),
             )?;
             values.extend_from_slice(value_col.chunks())
         }
@@ -434,28 +433,28 @@ mod test {
 
     #[test]
     #[cfg_attr(miri, ignore)]
-    fn test_melt() -> PolarsResult<()> {
+    fn test_unpivot() -> PolarsResult<()> {
         let df = df!("A" => &["a", "b", "a"],
          "B" => &[1, 3, 5],
          "C" => &[10, 11, 12],
          "D" => &[2, 4, 6]
         )
         .unwrap();
 
-        let melted = df.melt(["A", "B"], ["C", "D"])?;
+        let unpivoted = df.unpivot(["A", "B"], ["C", "D"])?;
         assert_eq!(
-            Vec::from(melted.column("value")?.i32()?),
+            Vec::from(unpivoted.column("value")?.i32()?),
             &[Some(10), Some(11), Some(12), Some(2), Some(4), Some(6)]
         );
 
-        let args = MeltArgs {
-            id_vars: vec![],
-            value_vars: vec![],
+        let args = UnpivotArgs {
+            index: vec![],
+            on: vec![],
             ..Default::default()
         };
 
-        let melted = df.melt2(args).unwrap();
-        let value = melted.column("value")?;
+        let unpivoted = df.unpivot2(args).unwrap();
+        let value = unpivoted.column("value")?;
         // String because of supertype
         let value = value.str()?;
         let value = value.into_no_null_iter().collect::<Vec<_>>();
@@ -464,22 +463,22 @@ mod test {
             &["a", "b", "a", "1", "3", "5", "10", "11", "12", "2", "4", "6"]
         );
 
-        let args = MeltArgs {
-            id_vars: vec!["A".into()],
-            value_vars: vec![],
+        let args = UnpivotArgs {
+            index: vec!["A".into()],
+            on: vec![],
             ..Default::default()
         };
 
-        let melted = df.melt2(args).unwrap();
-        let value = melted.column("value")?;
+        let unpivoted = df.unpivot2(args).unwrap();
+        let value = unpivoted.column("value")?;
         let value = value.i32()?;
         let value = value.into_no_null_iter().collect::<Vec<_>>();
         assert_eq!(value, &[1, 3, 5, 10, 11, 12, 2, 4, 6]);
-        let variable = melted.column("variable")?;
+        let variable = unpivoted.column("variable")?;
         let variable = variable.str()?;
         let variable = variable.into_no_null_iter().collect::<Vec<_>>();
         assert_eq!(variable, &["B", "B", "B", "C", "C", "C", "D", "D", "D"]);
-        assert!(melted.column("A").is_ok());
+        assert!(unpivoted.column("A").is_ok());
         Ok(())
     }
 }

crates/polars-core/src/prelude.rs

@@ -37,7 +37,7 @@ pub use crate::datatypes::{ArrayCollectIterExt, *};
 pub use crate::error::{
     polars_bail, polars_ensure, polars_err, polars_warn, PolarsError, PolarsResult,
 };
-pub use crate::frame::explode::MeltArgs;
+pub use crate::frame::explode::UnpivotArgs;
 #[cfg(feature = "algorithm_group_by")]
 pub(crate) use crate::frame::group_by::aggregations::*;
 #[cfg(feature = "algorithm_group_by")]

crates/polars-lazy/src/frame/mod.rs

@@ -1600,12 +1600,12 @@ impl LazyFrame {
         self.slice(neg_tail, n)
     }
 
-    /// Melt the DataFrame from wide to long format.
+    /// Unpivot the DataFrame from wide to long format.
     ///
-    /// See [`MeltArgs`] for information on how to melt a DataFrame.
-    pub fn melt(self, args: MeltArgs) -> LazyFrame {
+    /// See [`UnpivotArgs`] for information on how to unpivot a DataFrame.
+    pub fn unpivot(self, args: UnpivotArgs) -> LazyFrame {
         let opt_state = self.get_opt_state();
-        let lp = self.get_plan_builder().melt(args).build();
+        let lp = self.get_plan_builder().unpivot(args).build();
         Self::from_logical_plan(lp, opt_state)
     }
 

crates/polars-lazy/src/tests/queries.rs

@@ -46,18 +46,18 @@ fn test_lazy_alias() {
 }
 
 #[test]
-fn test_lazy_melt() {
+fn test_lazy_unpivot() {
     let df = get_df();
 
-    let args = MeltArgs {
-        id_vars: vec!["petal_width".into(), "petal_length".into()],
-        value_vars: vec!["sepal_length".into(), "sepal_width".into()],
+    let args = UnpivotArgs {
+        index: vec!["petal_width".into(), "petal_length".into()],
+        on: vec!["sepal_length".into(), "sepal_width".into()],
         ..Default::default()
     };
 
     let out = df
         .lazy()
-        .melt(args)
+        .unpivot(args)
         .filter(col("variable").eq(lit("sepal_length")))
         .select([col("variable"), col("petal_width"), col("value")])
         .collect()

crates/polars-pipe/src/pipeline/dispatcher/mod.rs

@@ -62,7 +62,7 @@ impl ThreadedSink {
 ///         succeed.
 ///         Think for example on multiply a few columns, or applying a predicate.
 ///         Operators can shrink the batches: filter
-///         Grow the batches: explode/ melt
+///         Grow the batches: explode/ unpivot
 ///         Keep them the same size: element-wise operations
 ///         The probe side of join operations is also an operator.
 ///

crates/polars-plan/src/plans/builder_dsl.rs

@@ -339,10 +339,10 @@ impl DslBuilder {
         .into()
     }
 
-    pub fn melt(self, args: MeltArgs) -> Self {
+    pub fn unpivot(self, args: UnpivotArgs) -> Self {
         DslPlan::MapFunction {
             input: Arc::new(self.0),
-            function: DslFunction::Melt { args },
+            function: DslFunction::Unpivot { args },
         }
         .into()
     }

crates/polars-plan/src/plans/builder_ir.rs

@@ -297,10 +297,10 @@ impl<'a> IRBuilder<'a> {
         self.add_alp(lp)
     }
 
-    pub fn melt(self, args: Arc<MeltArgs>) -> Self {
+    pub fn unpivot(self, args: Arc<UnpivotArgs>) -> Self {
         let lp = IR::MapFunction {
             input: self.root,
-            function: FunctionNode::Melt {
+            function: FunctionNode::Unpivot {
                 args,
                 schema: Default::default(),
             },

crates/polars-plan/src/plans/functions/dsl.rs

@@ -9,8 +9,8 @@ pub enum DslFunction {
     Explode {
         columns: Vec<Expr>,
     },
-    Melt {
-        args: MeltArgs,
+    Unpivot {
+        args: UnpivotArgs,
     },
     RowIndex {
         name: Arc<str>,
@@ -79,7 +79,7 @@ impl DslFunction {
                     schema: Default::default(),
                 }
             },
-            DslFunction::Melt { args } => FunctionNode::Melt {
+            DslFunction::Unpivot { args } => FunctionNode::Unpivot {
                 args: Arc::new(args),
                 schema: Default::default(),
             },
@@ -125,7 +125,7 @@ impl Display for DslFunction {
         match self {
             FunctionNode(inner) => write!(f, "{inner}"),
             Explode { .. } => write!(f, "EXPLODE"),
-            Melt { .. } => write!(f, "MELT"),
+            Unpivot { .. } => write!(f, "UNPIVOT"),
             RowIndex { .. } => write!(f, "WITH ROW INDEX"),
             Stats(_) => write!(f, "STATS"),
             FillNan(_) => write!(f, "FILL NAN"),