feat: Add cat.len_chars and cat.len_bytes

crates/polars-plan/src/dsl/cat.rs

@@ -8,4 +8,16 @@ impl CategoricalNameSpace {
         self.0
             .apply_private(CategoricalFunction::GetCategories.into())
     }
+
+    #[cfg(feature = "strings")]
+    pub fn len_bytes(self) -> Expr {
+        self.0
+            .map_private(FunctionExpr::Categorical(CategoricalFunction::LenBytes))
+    }
+
+    #[cfg(feature = "strings")]
+    pub fn len_chars(self) -> Expr {
+        self.0
+            .map_private(FunctionExpr::Categorical(CategoricalFunction::LenChars))
+    }
 }

crates/polars-plan/src/dsl/function_expr/cat.rs

@@ -5,13 +5,21 @@ use crate::map;
 #[derive(Clone, PartialEq, Debug, Eq, Hash)]
 pub enum CategoricalFunction {
     GetCategories,
+    #[cfg(feature = "strings")]
+    LenBytes,
+    #[cfg(feature = "strings")]
+    LenChars,
 }
 
 impl CategoricalFunction {
     pub(super) fn get_field(&self, mapper: FieldsMapper) -> PolarsResult<Field> {
         use CategoricalFunction::*;
         match self {
             GetCategories => mapper.with_dtype(DataType::String),
+            #[cfg(feature = "strings")]
+            LenBytes => mapper.with_dtype(DataType::UInt32),
+            #[cfg(feature = "strings")]
+            LenChars => mapper.with_dtype(DataType::UInt32),
         }
     }
 }
@@ -21,6 +29,10 @@ impl Display for CategoricalFunction {
         use CategoricalFunction::*;
         let s = match self {
             GetCategories => "get_categories",
+            #[cfg(feature = "strings")]
+            LenBytes => "len_bytes",
+            #[cfg(feature = "strings")]
+            LenChars => "len_chars",
         };
         write!(f, "cat.{s}")
     }
@@ -31,6 +43,10 @@ impl From<CategoricalFunction> for SpecialEq<Arc<dyn ColumnsUdf>> {
         use CategoricalFunction::*;
         match func {
             GetCategories => map!(get_categories),
+            #[cfg(feature = "strings")]
+            LenBytes => map!(len_bytes),
+            #[cfg(feature = "strings")]
+            LenChars => map!(len_chars),
         }
     }
 }
@@ -48,3 +64,42 @@ fn get_categories(s: &Column) -> PolarsResult<Column> {
     let arr = rev_map.get_categories().clone().boxed();
     Series::try_from((ca.name().clone(), arr)).map(Column::from)
 }
+
+/// Apply a function to the categories of a categorical column and re-broadcast the result back to
+/// to the array.
+fn apply_to_cats<F, T>(s: &Column, mut op: F) -> PolarsResult<Column>
+where
+    F: FnMut(&StringChunked) -> ChunkedArray<T>,
+    ChunkedArray<T>: IntoSeries,
+    T: PolarsDataType,
+{
+    let ca = s.categorical()?;
+    let (categories, phys) = match &**ca.get_rev_map() {
+        RevMapping::Local(c, _) => (c, ca.physical().cast(&IDX_DTYPE)?),
+        RevMapping::Global(physical_map, c, _) => {
+            // Map physical to its local representation for use with take() later.
+            let phys = ca
+                .physical()
+                .apply(|opt_v| opt_v.map(|v| *physical_map.get(&v).unwrap()));
+            let out = phys.cast(&IDX_DTYPE)?;
+            (c, out)
+        },
+    };
+
+    // Apply function to categories
+    let categories = StringChunked::with_chunk(PlSmallStr::EMPTY, categories.clone());
+    let result = op(&categories).into_series();
+
+    let out = result.take(phys.idx()?)?;
+    Ok(out.into_column())
+}
+
+#[cfg(feature = "strings")]
+fn len_bytes(s: &Column) -> PolarsResult<Column> {
+    apply_to_cats(s, |s| s.str_len_bytes())
+}
+
+#[cfg(feature = "strings")]
+fn len_chars(s: &Column) -> PolarsResult<Column> {
+    apply_to_cats(s, |s| s.str_len_chars())
+}

crates/polars-python/src/expr/categorical.rs

@@ -7,4 +7,12 @@ impl PyExpr {
     fn cat_get_categories(&self) -> Self {
         self.inner.clone().cat().get_categories().into()
     }
+
+    fn cat_len_bytes(&self) -> Self {
+        self.inner.clone().cat().len_bytes().into()
+    }
+
+    fn cat_len_chars(&self) -> Self {
+        self.inner.clone().cat().len_chars().into()
+    }
 }

crates/polars-python/src/lazyframe/visitor/expr_nodes.rs

@@ -8,7 +8,7 @@ use polars_ops::series::InterpolationMethod;
 use polars_ops::series::SearchSortedSide;
 use polars_plan::dsl::function_expr::rolling::RollingFunction;
 use polars_plan::dsl::function_expr::rolling_by::RollingFunctionBy;
-use polars_plan::dsl::{BooleanFunction, StringFunction, TemporalFunction};
+use polars_plan::dsl::{BooleanFunction, CategoricalFunction, StringFunction, TemporalFunction};
 use polars_plan::prelude::{
     AExpr, FunctionExpr, GroupbyOptions, IRAggExpr, LiteralValue, Operator, PowFunction,
     WindowMapping, WindowType,
@@ -171,6 +171,21 @@ impl PyStringFunction {
     }
 }
 
+#[pyclass(name = "CategoricalFunction", eq)]
+#[derive(Copy, Clone, PartialEq)]
+pub enum PyCategoricalFunction {
+    GetCategories,
+    LenBytes,
+    LenChars,
+}
+
+#[pymethods]
+impl PyCategoricalFunction {
+    fn __hash__(&self) -> isize {
+        *self as isize
+    }
+}
+
 #[pyclass(name = "BooleanFunction", eq)]
 #[derive(Copy, Clone, PartialEq)]
 pub enum PyBooleanFunction {
@@ -793,8 +808,16 @@ pub(crate) fn into_py(py: Python<'_>, expr: &AExpr) -> PyResult<PyObject> {
                 FunctionExpr::BinaryExpr(_) => {
                     return Err(PyNotImplementedError::new_err("binary expr"))
                 },
-                FunctionExpr::Categorical(_) => {
-                    return Err(PyNotImplementedError::new_err("categorical expr"))
+                FunctionExpr::Categorical(catfun) => match catfun {
+                    CategoricalFunction::GetCategories => {
+                        (PyCategoricalFunction::GetCategories.into_py(py),).to_object(py)
+                    },
+                    CategoricalFunction::LenBytes => {
+                        (PyCategoricalFunction::LenBytes.into_py(py),).to_object(py)
+                    },
+                    CategoricalFunction::LenChars => {
+                        (PyCategoricalFunction::LenChars.into_py(py),).to_object(py)
+                    },
                 },
                 FunctionExpr::ListExpr(_) => {
                     return Err(PyNotImplementedError::new_err("list expr"))

py-polars/docs/source/reference/expressions/categories.rst

@@ -10,3 +10,5 @@ The following methods are available under the `expr.cat` attribute.
    :template: autosummary/accessor_method.rst
 
     Expr.cat.get_categories
+    Expr.cat.len_bytes
+    Expr.cat.len_chars

py-polars/docs/source/reference/series/categories.rst

@@ -11,5 +11,7 @@ The following methods are available under the `Series.cat` attribute.
 
     Series.cat.get_categories
     Series.cat.is_local
+    Series.cat.len_bytes
+    Series.cat.len_chars
     Series.cat.to_local
     Series.cat.uses_lexical_ordering

py-polars/polars/expr/categorical.py

@@ -38,3 +38,94 @@ def get_categories(self) -> Expr:
         └──────┘
         """
         return wrap_expr(self._pyexpr.cat_get_categories())
+
+    def len_bytes(self) -> Expr:
+        """
+        Return the byte-length of the string representation of each value.
+
+        Returns
+        -------
+        Expr
+            Expression of data type :class:`UInt32`.
+
+        See Also
+        --------
+        len_chars
+
+        Notes
+        -----
+        When working with non-ASCII text, the length in bytes is not the same as the
+        length in characters. You may want to use :func:`len_chars` instead.
+        Note that :func:`len_bytes` is much more performant (_O(1)_) than
+        :func:`len_chars` (_O(n)_).
+
+        Examples
+        --------
+        >>> df = pl.DataFrame(
+        ...     {"a": pl.Series(["Café", "345", "東京", None], dtype=pl.Categorical)}
+        ... )
+        >>> df.with_columns(
+        ...     pl.col("a").cat.len_bytes().alias("n_bytes"),
+        ...     pl.col("a").cat.len_chars().alias("n_chars"),
+        ... )
+        shape: (4, 3)
+        ┌──────┬─────────┬─────────┐
+        │ a    ┆ n_bytes ┆ n_chars │
+        │ ---  ┆ ---     ┆ ---     │
+        │ cat  ┆ u32     ┆ u32     │
+        ╞══════╪═════════╪═════════╡
+        │ Café ┆ 5       ┆ 4       │
+        │ 345  ┆ 3       ┆ 3       │
+        │ 東京 ┆ 6       ┆ 2       │
+        │ null ┆ null    ┆ null    │
+        └──────┴─────────┴─────────┘
+        """
+        return wrap_expr(self._pyexpr.cat_len_bytes())
+
+    def len_chars(self) -> Expr:
+        """
+        Return the number of characters of the string representation of each value.
+
+        Returns
+        -------
+        Expr
+            Expression of data type :class:`UInt32`.
+
+        See Also
+        --------
+        len_bytes
+
+        Notes
+        -----
+        When working with ASCII text, use :func:`len_bytes` instead to achieve
+        equivalent output with much better performance:
+        :func:`len_bytes` runs in _O(1)_, while :func:`len_chars` runs in (_O(n)_).
+
+        A character is defined as a `Unicode scalar value`_. A single character is
+        represented by a single byte when working with ASCII text, and a maximum of
+        4 bytes otherwise.
+
+        .. _Unicode scalar value: https://www.unicode.org/glossary/#unicode_scalar_value
+
+        Examples
+        --------
+        >>> df = pl.DataFrame(
+        ...     {"a": pl.Series(["Café", "345", "東京", None], dtype=pl.Categorical)}
+        ... )
+        >>> df.with_columns(
+        ...     pl.col("a").cat.len_chars().alias("n_chars"),
+        ...     pl.col("a").cat.len_bytes().alias("n_bytes"),
+        ... )
+        shape: (4, 3)
+        ┌──────┬─────────┬─────────┐
+        │ a    ┆ n_chars ┆ n_bytes │
+        │ ---  ┆ ---     ┆ ---     │
+        │ cat  ┆ u32     ┆ u32     │
+        ╞══════╪═════════╪═════════╡
+        │ Café ┆ 4       ┆ 5       │
+        │ 345  ┆ 3       ┆ 3       │
+        │ 東京 ┆ 2       ┆ 6       │
+        │ null ┆ null    ┆ null    │
+        └──────┴─────────┴─────────┘
+        """
+        return wrap_expr(self._pyexpr.cat_len_chars())

py-polars/polars/series/categorical.py

@@ -112,3 +112,76 @@ def uses_lexical_ordering(self) -> bool:
         True
         """
         return self._s.cat_uses_lexical_ordering()
+
+    def len_bytes(self) -> Series:
+        """
+        Return the byte-length of the string representation of each value.
+
+        Returns
+        -------
+        Series
+            Series of data type :class:`UInt32`.
+
+        See Also
+        --------
+        len_chars
+
+        Notes
+        -----
+        When working with non-ASCII text, the length in bytes is not the same as the
+        length in characters. You may want to use :func:`len_chars` instead.
+        Note that :func:`len_bytes` is much more performant (_O(1)_) than
+        :func:`len_chars` (_O(n)_).
+
+        Examples
+        --------
+        >>> s = pl.Series(["Café", "345", "東京", None], dtype=pl.Categorical)
+        >>> s.cat.len_bytes()
+        shape: (4,)
+        Series: '' [u32]
+        [
+            5
+            3
+            6
+            null
+        ]
+        """
+
+    def len_chars(self) -> Series:
+        """
+        Return the number of characters of the string representation of each value.
+
+        Returns
+        -------
+        Series
+            Series of data type :class:`UInt32`.
+
+        See Also
+        --------
+        len_bytes
+
+        Notes
+        -----
+        When working with ASCII text, use :func:`len_bytes` instead to achieve
+        equivalent output with much better performance:
+        :func:`len_bytes` runs in _O(1)_, while :func:`len_chars` runs in (_O(n)_).
+
+        A character is defined as a `Unicode scalar value`_. A single character is
+        represented by a single byte when working with ASCII text, and a maximum of
+        4 bytes otherwise.
+
+        .. _Unicode scalar value: https://www.unicode.org/glossary/#unicode_scalar_value
+
+        Examples
+        --------
+        >>> s = pl.Series(["Café", "345", "東京", None], dtype=pl.Categorical)
+        >>> s.cat.len_chars()
+        shape: (4,)
+        Series: '' [u32]
+        [
+            4
+            3
+            2
+            null
+        ]
+        """