Merge pull request #192 from DeaMariaLeon/when

Improve error message for is_in

narwhals/expression.py

@@ -844,7 +844,12 @@ def is_between(
         )
 
     def is_in(self, other: Any) -> Expr:
-        return self.__class__(lambda plx: self._call(plx).is_in(other))
+        if isinstance(other, Iterable) and not isinstance(other, (str, bytes)):
+            return self.__class__(lambda plx: self._call(plx).is_in(other))
+        else:
+            raise NotImplementedError(
+                "Narwhals `is_in` doesn't accept expressions as an argument, as opposed to Polars. You should provide an iterable instead."
+            )
 
     def filter(self, other: Any) -> Expr:
         return self.__class__(

tests/test_series.py

@@ -59,6 +59,18 @@ def test_is_in(df_raw: Any) -> None:
     assert result[2]
 
 
+@pytest.mark.parametrize("df_raw", [df_pandas, df_polars])
+@pytest.mark.filterwarnings("ignore:np.find_common_type is deprecated:DeprecationWarning")
+def test_is_in_other(df_raw: Any) -> None:
+    with pytest.raises(
+        NotImplementedError,
+        match=(
+            "Narwhals `is_in` doesn't accept expressions as an argument, as opposed to Polars. You should provide an iterable instead."
+        ),
+    ):
+        nw.from_native(df_raw).with_columns(contains=nw.col("c").is_in("sets"))
+
+
 @pytest.mark.parametrize("df_raw", [df_pandas, df_polars])
 @pytest.mark.filterwarnings("ignore:np.find_common_type is deprecated:DeprecationWarning")
 def test_filter(df_raw: Any) -> None: