feat: Raise on mismatched *Frame joins

narwhals/dataframe.py

@@ -38,6 +38,7 @@
     is_list_of,
     is_sequence_like,
     is_slice_none,
+    qualified_type_name,
     supports_arrow_c_stream,
     zip_strict,
 )
@@ -139,6 +140,12 @@ def _flatten_and_extract(
     def _extract_compliant(self, arg: Any) -> Any:
         raise NotImplementedError
 
+    def _extract_compliant_frame(self, other: Self | Any, /) -> Any:
+        if isinstance(other, type(self)):
+            return other._compliant_frame
+        msg = f"Expected `other` to be a {qualified_type_name(self)!r}, got: {qualified_type_name(other)!r}"
+        raise TypeError(msg)
+
     def _check_columns_exist(self, subset: Sequence[str]) -> ColumnNotFoundError | None:
         return check_columns_exist(subset, available=self.columns)
 
@@ -269,7 +276,7 @@ def join(
         left_on = [left_on] if isinstance(left_on, str) else left_on
         right_on = [right_on] if isinstance(right_on, str) else right_on
         compliant = self._compliant_frame
-        other = self._extract_compliant(other)
+        other = self._extract_compliant_frame(other)
 
         if how not in _supported_joins:
             msg = f"Only the following join strategies are supported: {_supported_joins}; found '{how}'."
@@ -357,7 +364,7 @@ def join_asof(
 
         return self._with_compliant(
             self._compliant_frame.join_asof(
-                self._extract_compliant(other),
+                self._extract_compliant_frame(other),
                 left_on=left_on,
                 right_on=right_on,
                 by_left=by_left,
@@ -448,8 +455,6 @@ def _extract_compliant(self, arg: Any) -> Any:
         from narwhals.series import Series
 
         plx: EagerNamespaceAny = self.__narwhals_namespace__()
-        if isinstance(arg, BaseFrame):
-            return arg._compliant_frame
         if isinstance(arg, Series):
             return arg._compliant_series._to_expr()
         if isinstance(arg, Expr):
@@ -2296,8 +2301,6 @@ def _extract_compliant(self, arg: Any) -> Any:
         from narwhals.expr import Expr
         from narwhals.series import Series
 
-        if isinstance(arg, BaseFrame):
-            return arg._compliant_frame
         if isinstance(arg, Series):  # pragma: no cover
             msg = "Binary operations between Series and LazyFrame are not supported."
             raise TypeError(msg)

narwhals/stable/v1/__init__.py

@@ -237,12 +237,9 @@ def _dataframe(self) -> type[DataFrame[Any]]:
     def _extract_compliant(self, arg: Any) -> Any:
         # After v1, we raise when passing order-dependent or length-changing
         # expressions to LazyFrame
-        from narwhals.dataframe import BaseFrame
         from narwhals.expr import Expr
         from narwhals.series import Series
 
-        if isinstance(arg, BaseFrame):
-            return arg._compliant_frame
         if isinstance(arg, Series):  # pragma: no cover
             msg = "Mixing Series with LazyFrame is not supported."
             raise TypeError(msg)

tests/frame/join_test.py

@@ -798,3 +798,23 @@ def test_join_duplicate_column_names(
     else:
         with pytest.raises(exception):
             df.join(df, on=["a"]).join(df, on=["a"])
+
+
+def test_join_same_laziness(constructor: Constructor) -> None:
+    pytest.importorskip("polars")
+    import polars as pl
+
+    data_left = {"id": [1, 2, 3], "age": [25, 30, 35]}
+    data_right = {"id": [2, 3, 4], "active": [False, True, True]}
+    frame = nw.from_native(constructor(data_left))
+    df_pl = pl.DataFrame(data_right)
+    frame_pl: pl.DataFrame | pl.LazyFrame
+    if isinstance(frame, nw.DataFrame):
+        msg = r"Expected.+\.DataFrame.+got.+\.LazyFrame"
+        frame_pl = df_pl.lazy()
+    else:
+        msg = r"Expected.+\.LazyFrame.+got.+\.DataFrame"
+        frame_pl = df_pl
+    other = nw.from_native(frame_pl)
+    with pytest.raises(TypeError, match=msg):
+        frame.join(other, on="id")  # type: ignore[arg-type]