fix[python]: ensure is_in empty list has dtype null (#4562)

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

@@ -3269,7 +3269,7 @@ def pow(self, exponent: int | float | pli.Series | Expr) -> Expr:
         exponent = expr_to_lit_or_expr(exponent)
         return wrap_expr(self._pyexpr.pow(exponent._pyexpr))
 
-    def is_in(self, other: Expr | Sequence[Any] | str) -> Expr:
+    def is_in(self, other: Expr | Sequence[Any] | str | pli.Series) -> Expr:
         """
         Check if elements of this expression are present in the other Series.
 
@@ -3303,7 +3303,10 @@ def is_in(self, other: Expr | Sequence[Any] | str) -> Expr:
 
         """
         if isinstance(other, Sequence) and not isinstance(other, str):
-            other = pli.lit(pli.Series(other))
+            if len(other) == 0:
+                other = pli.lit(None)
+            else:
+                other = pli.lit(pli.Series(other))
         else:
             other = expr_to_lit_or_expr(other, str_to_lit=False)
         return wrap_expr(self._pyexpr.is_in(other._pyexpr))

py-polars/polars/internals/series/series.py

@@ -1913,7 +1913,7 @@ def is_not_nan(self) -> Series:
         """
         return wrap_s(self._s.is_not_nan())
 
-    def is_in(self, other: Series | Sequence[object]) -> Series:
+    def is_in(self, other: Series | Sequence[Any]) -> Series:
         """
         Check if elements of this Series are in the other Series, or
         if this Series is itself a member of the other Series.
@@ -1963,11 +1963,7 @@ def is_in(self, other: Series | Sequence[object]) -> Series:
         ]
 
         """
-        if isinstance(other, str):
-            raise TypeError("'other' parameter expects non-string sequence data")
-        elif isinstance(other, Sequence):
-            other = Series("", other)
-        return wrap_s(self._s.is_in(other._s))
+        return self.to_frame().select(pli.col(self.name).is_in(other)).to_series()
 
     def arg_true(self) -> Series:
         """

py-polars/src/series.rs

@@ -892,17 +892,6 @@ impl PySeries {
         })
     }
 
-    #[cfg(feature = "is_in")]
-    pub fn is_in(&self, py: Python, other: &PySeries) -> PyResult<Self> {
-        py.allow_threads(|| {
-            let out = self
-                .series
-                .is_in(&other.series)
-                .map_err(PyPolarsErr::from)?;
-            Ok(out.into_series().into())
-        })
-    }
-
     pub fn clone(&self) -> Self {
         PySeries::new(self.series.clone())
     }

py-polars/tests/test_lists.py

@@ -430,3 +430,7 @@ def test_list_eval_type_coercion() -> None:
             .alias("col_last")
         ]
     ).to_dict(False) == {"col_last": [[3]]}
+
+
+def test_is_empty_list_4559() -> None:
+    assert pl.Series(["a"]).is_in([]).to_list() == [False]