fix[python]: enforce strict rules in expr.apply

polars/polars-core/src/utils/mod.rs

@@ -670,11 +670,10 @@ fn _get_supertype(l: &DataType, r: &DataType) -> Option<DataType> {
             #[cfg(all(feature = "dtype-date", feature = "dtype-time"))]
             (Date, Time) => Some(Int64),
 
-            // everything can be cast to a string
-            (_, Utf8) => Some(Utf8),
+            // every known type can be casted to a string
+            (dt, Utf8) if dt != &DataType::Unknown => Some(Utf8),
 
             (dt, Null) => Some(dt.clone()),
-            (Null, dt) => Some(dt.clone()),
 
             #[cfg(all(feature = "dtype-duration", feature = "dtype-datetime"))]
             (Duration(lu), Datetime(ru, Some(tz))) | (Datetime(lu, Some(tz)), Duration(ru)) => {
@@ -719,7 +718,7 @@ fn _get_supertype(l: &DataType, r: &DataType) -> Option<DataType> {
                 let st = _get_supertype(inner, other)?;
                 Some(DataType::List(Box::new(st)))
             }
-            (dt, Unknown) => Some(dt.clone()),
+            (_, Unknown) => Some(Unknown),
             _ => None,
         }
     }

polars/polars-lazy/src/dsl/dt.rs

@@ -64,6 +64,23 @@ impl DateLikeNameSpace {
         )
     }
 
+    /// Change the underlying [`TimeZone`] of the [`Series`]. This does not modify the data.
+    pub fn with_time_zone(self, tz: Option<TimeZone>) -> Expr {
+        self.0.map(
+            move |s| match s.dtype() {
+                DataType::Datetime(_, _) => {
+                    let mut ca = s.datetime().unwrap().clone();
+                    ca.set_time_zone(tz.clone());
+                    Ok(ca.into_series())
+                }
+                dt => Err(PolarsError::ComputeError(
+                    format!("Series of dtype {:?} has got no time zone", dt).into(),
+                )),
+            },
+            GetOutput::same_type(),
+        )
+    }
+
     /// Get the year of a Date/Datetime
     pub fn year(self) -> Expr {
         let function = move |s: Series| s.year().map(|ca| ca.into_series());

polars/polars-lazy/src/logical_plan/optimizer/type_coercion.rs

@@ -185,8 +185,10 @@ impl OptimizationRule for TypeCoercionRule {
             } => {
                 let input_schema = get_schema(lp_arena, lp_node);
                 let (left, type_left) = get_aexpr_and_type(expr_arena, node_left, &input_schema)?;
+
                 let (right, type_right) =
                     get_aexpr_and_type(expr_arena, node_right, &input_schema)?;
+                early_escape(&type_left, &type_right)?;
 
                 // don't coerce string with number comparisons. They must error
                 match (&type_left, &type_right, op) {
@@ -449,9 +451,6 @@ impl OptimizationRule for TypeCoercionRule {
                         get_aexpr_and_type(expr_arena, *other, &input_schema)?;
 
                     // early return until Unknown is set
-                    if let DataType::Unknown = &type_other {
-                        return None;
-                    }
                     early_escape(&super_type, &type_other)?;
                     let new_st = get_supertype(&super_type, &type_other).ok()?;
                     super_type = modify_supertype(new_st, self_ae, other, &type_self, &type_other)

py-polars/docs/source/reference/datatypes.rst

@@ -56,6 +56,7 @@ Other
     Null
     Object
     Utf8
+    Unknown
 
 Functions
 ~~~~~~~~~

py-polars/polars/__init__.py

@@ -44,6 +44,7 @@ def version() -> str:
     UInt16,
     UInt32,
     UInt64,
+    Unknown,
     Utf8,
     get_idx_type,
 )
@@ -169,6 +170,7 @@ def version() -> str:
     "Field",
     "Struct",
     "Null",
+    "Unknown",
     "PolarsDataType",
     "get_idx_type",
     # polars.io

py-polars/polars/datatypes.py

@@ -138,6 +138,10 @@ class Null(DataType):
     """Type representing Null / None values."""
 
 
+class Unknown(DataType):
+    """Type representing Datatype values that could not be determined statically."""
+
+
 class List(DataType):
     def __init__(self, inner: type[DataType]):
         """

py-polars/polars/internals/expr/datetime.py

@@ -4,7 +4,7 @@
 from typing import TYPE_CHECKING
 
 import polars.internals as pli
-from polars.datatypes import DTYPE_TEMPORAL_UNITS, Date, Datetime, Int32
+from polars.datatypes import DTYPE_TEMPORAL_UNITS, Date, Int32
 from polars.utils import _timedelta_to_pl_duration
 
 if TYPE_CHECKING:
@@ -879,9 +879,7 @@ def with_time_zone(self, tz: str | None) -> pli.Expr:
         └─────────────────────┴─────────────────────────────┘
 
         """
-        return pli.wrap_expr(self._pyexpr).map(
-            lambda s: s.dt.with_time_zone(tz), return_dtype=Datetime
-        )
+        return pli.wrap_expr(self._pyexpr.dt_with_time_zone(tz))
 
     def days(self) -> pli.Expr:
         """

py-polars/polars/internals/expr/expr.py

@@ -3025,6 +3025,8 @@ def apply(
             Lambda/ function to apply.
         return_dtype
             Dtype of the output Series.
+            If not set, polars will assume that
+            the dtype remains unchanged.
 
         Examples
         --------

py-polars/src/conversion.rs

@@ -278,7 +278,7 @@ impl ToPyObject for Wrap<DataType> {
                 struct_class.call1((fields,)).unwrap().into()
             }
             DataType::Null => pl.getattr("Null").unwrap().into(),
-            dt => panic!("{} not supported", dt),
+            DataType::Unknown => pl.getattr("Unknown").unwrap().into(),
         }
     }
 }
@@ -321,6 +321,7 @@ impl FromPyObject<'_> for Wrap<DataType> {
                     "Object" => DataType::Object("unknown"),
                     "List" => DataType::List(Box::new(DataType::Boolean)),
                     "Null" => DataType::Null,
+                    "Unknown" => DataType::Unknown,
                     dt => panic!("{} not expected as Python type for dtype conversion", dt),
                 }
             }

py-polars/src/lazy/apply.rs

@@ -4,6 +4,7 @@ use pyo3::types::PyList;
 
 use crate::lazy::dsl::PyExpr;
 use crate::prelude::PyDataType;
+use crate::py_modules::POLARS;
 use crate::series::PySeries;
 
 trait ToSeries {
@@ -117,29 +118,38 @@ pub(crate) fn binary_lambda(lambda: &PyObject, a: Series, b: Series) -> Result<S
 
 pub fn map_single(
     pyexpr: &PyExpr,
-    py: Python,
     lambda: PyObject,
     output_type: &PyAny,
     agg_list: bool,
 ) -> PyExpr {
     let output_type = get_output_type(output_type);
-    // get the pypolars module
-    // do the import outside of the function to prevent import side effects in a hot loop.
-    let pypolars = PyModule::import(py, "polars").unwrap().to_object(py);
 
+    let output_type2 = output_type.clone();
     let function = move |s: Series| {
         Python::with_gil(|py| {
+            let output_type = output_type2.clone().unwrap_or_else(|| DataType::Unknown);
+
             // this is a python Series
-            let out = call_lambda_with_series(py, s.clone(), &lambda, &pypolars)
+            let out = call_lambda_with_series(py, s.clone(), &lambda, &POLARS)
                 .map_err(|e| PolarsError::ComputeError(format!("{e}").into()))?;
+            let s = out.to_series(py, &POLARS, s.name());
 
-            Ok(out.to_series(py, &pypolars, s.name()))
+            if !matches!(output_type, DataType::Unknown) && s.dtype() != &output_type {
+                Err(PolarsError::SchemaMisMatch(
+                    format!("Expected output type: '{:?}', but got '{:?}'. Set 'return_dtype' to the proper datatype.", output_type, s.dtype()).into()))
+            } else {
+                Ok(s)
+            }
         })
     };
 
     let output_map = GetOutput::map_field(move |fld| match output_type {
         Some(ref dt) => Field::new(fld.name(), dt.clone()),
-        None => fld.clone(),
+        None => {
+            let mut fld = fld.clone();
+            fld.coerce(DataType::Unknown);
+            fld
+        }
     });
     if agg_list {
         pyexpr.clone().inner.map_list(function, output_map).into()

py-polars/src/lazy/dsl.rs

@@ -872,6 +872,10 @@ impl PyExpr {
         self.inner.clone().dt().with_time_unit(tu.0).into()
     }
 
+    pub fn dt_with_time_zone(&self, tz: Option<TimeZone>) -> PyExpr {
+        self.inner.clone().dt().with_time_zone(tz).into()
+    }
+
     pub fn dt_cast_time_unit(&self, tu: Wrap<TimeUnit>) -> PyExpr {
         self.inner.clone().dt().cast_time_unit(tu.0).into()
     }
@@ -1010,8 +1014,8 @@ impl PyExpr {
             .into()
     }
 
-    pub fn map(&self, py: Python, lambda: PyObject, output_type: &PyAny, agg_list: bool) -> PyExpr {
-        map_single(self, py, lambda, output_type, agg_list)
+    pub fn map(&self, lambda: PyObject, output_type: &PyAny, agg_list: bool) -> PyExpr {
+        map_single(self, lambda, output_type, agg_list)
     }
 
     pub fn dot(&self, other: PyExpr) -> PyExpr {

py-polars/tests/test_lazy.py

@@ -169,8 +169,12 @@ def test_apply_custom_function() -> None:
         .groupby("fruits")
         .agg(
             [
-                pl.col("cars").apply(lambda groups: groups.len()).alias("custom_1"),
-                pl.col("cars").apply(lambda groups: groups.len()).alias("custom_2"),
+                pl.col("cars")
+                .apply(lambda groups: groups.len(), return_dtype=pl.Int64)
+                .alias("custom_1"),
+                pl.col("cars")
+                .apply(lambda groups: groups.len(), return_dtype=pl.Int64)
+                .alias("custom_2"),
                 pl.count("cars").alias("cars_count"),
             ]
         )

py-polars/tests/test_schema.py

@@ -148,3 +148,22 @@ def test_join_as_of_by_schema() -> None:
     b = pl.DataFrame({"a": [1], "b": [2], "d": [4]}).lazy()
     q = a.join_asof(b, on="a", by="b")
     assert q.collect().columns == q.columns
+
+
+def test_unknown_apply() -> None:
+    df = pl.DataFrame(
+        {"Amount": [10, 1, 1, 5], "Flour": ["1000g", "100g", "50g", "75g"]}
+    )
+
+    q = df.lazy().select(
+        [
+            pl.col("Amount"),
+            pl.col("Flour").apply(lambda x: 100.0) / pl.col("Amount"),
+        ]
+    )
+
+    assert q.collect().to_dict(False) == {
+        "Amount": [10, 1, 1, 5],
+        "Flour": [10.0, 100.0, 100.0, 20.0],
+    }
+    assert q.dtypes == [pl.Int64, pl.Unknown]