From 0e26ef657388f827f71511b160d98a8cf1828d6e Mon Sep 17 00:00:00 2001
From: Francesco Bruzzesi <42817048+FBruzzesi@users.noreply.github.com>
Date: Mon, 23 Sep 2024 23:10:18 +0200
Subject: [PATCH] fix: move `__len__` to `DataFrame` only (#1053)

---
 narwhals/dataframe.py   | 6 +++---
 tests/frame/len_test.py | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/narwhals/dataframe.py b/narwhals/dataframe.py
index 83ebdcdc5..581c8617d 100644
--- a/narwhals/dataframe.py
+++ b/narwhals/dataframe.py
@@ -41,9 +41,6 @@ class BaseFrame(Generic[FrameT]):
     _compliant_frame: Any
     _level: Literal["full", "interchange"]
 
-    def __len__(self) -> Any:
-        return self._compliant_frame.__len__()
-
     def __native_namespace__(self) -> Any:
         return self._compliant_frame.__native_namespace__()
 
@@ -316,6 +313,9 @@ def __init__(
             msg = f"Expected an object which implements `__narwhals_dataframe__`, got: {type(df)}"
             raise AssertionError(msg)
 
+    def __len__(self) -> Any:
+        return self._compliant_frame.__len__()
+
     def __array__(self, dtype: Any = None, copy: bool | None = None) -> np.ndarray:
         return self._compliant_frame.__array__(dtype, copy=copy)
 
diff --git a/tests/frame/len_test.py b/tests/frame/len_test.py
index 6cb9269de..c06884e03 100644
--- a/tests/frame/len_test.py
+++ b/tests/frame/len_test.py
@@ -9,5 +9,5 @@
 
 
 def test_len(constructor_eager: Any) -> None:
-    result = len(nw.from_native(constructor_eager(data)))
+    result = len(nw.from_native(constructor_eager(data), eager_only=True))
     assert result == 4