feat: raise informative error message for DataFrame.__eq__/__neq__

narwhals/dataframe.py

@@ -311,6 +311,28 @@ def unpivot(
             )
         )
 
+    def __neq__(self, other: Any) -> NoReturn:
+        msg = (
+            "DataFrame.__eq__ and LazyFrame.__eq__ are not implemented, please "
+            "use expressions instead.\n\n"
+            "Hint: instead of\n"
+            "    df == 0\n"
+            "you may want to use\n"
+            "    df.select(nw.all() == 0)"
+        )
+        raise NotImplementedError(msg)
+
+    def __eq__(self, other: object) -> NoReturn:
+        msg = (
+            "DataFrame.__eq__ and LazyFrame.__eq__ are not implemented, please "
+            "use expressions instead.\n\n"
+            "Hint: instead of\n"
+            "    df == 0\n"
+            "you may want to use\n"
+            "    df.select(nw.all() == 0)"
+        )
+        raise NotImplementedError(msg)
+
 
 class DataFrame(BaseFrame[DataFrameT]):
     """Narwhals DataFrame, backed by a native dataframe.

tests/frame/eq_test.py

@@ -0,0 +1,17 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+
+import narwhals as nw
+
+if TYPE_CHECKING:
+    from tests.utils import Constructor
+
+
+def test_eq_neq_raise(constructor: Constructor) -> None:
+    with pytest.raises(NotImplementedError, match="please use expressions"):
+        nw.from_native(constructor({"a": [1, 2, 3]})) == 0  # noqa: B015
+    with pytest.raises(NotImplementedError, match="please use expressions"):
+        nw.from_native(constructor({"a": [1, 2, 3]})) != 0  # noqa: B015