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.__neq__ and LazyFrame.__neq__ 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

tests/translate/from_native_test.py

@@ -185,7 +185,7 @@ def test_pandas_like_validate() -> None:
 def test_init_already_narwhals() -> None:
     df = nw.from_native(pl.DataFrame({"a": [1, 2, 3]}))
     result = nw.from_native(df)
-    assert result is df  # type: ignore[comparison-overlap]
+    assert result is df
     s = df["a"]
     result_s = nw.from_native(s, allow_series=True)
     assert result_s is s
@@ -194,7 +194,7 @@ def test_init_already_narwhals() -> None:
 def test_init_already_narwhals_unstable() -> None:
     df = unstable_nw.from_native(pl.DataFrame({"a": [1, 2, 3]}))
     result = unstable_nw.from_native(df)
-    assert result is df  # type: ignore[comparison-overlap]
+    assert result is df
     s = df["a"]
     result_s = unstable_nw.from_native(s, allow_series=True)
     assert result_s is s
@@ -257,4 +257,4 @@ def __dataframe__(self) -> None:  # pragma: no cover
 
     mockdf = MockDf()
     result = nw.from_native(mockdf, eager_only=True, strict=False)
-    assert result is mockdf  # type: ignore[comparison-overlap]
+    assert result is mockdf