better error message

narwhals-dev · May 8, 2024 · 5a55ddc · 5a55ddc
1 parent ff70991
commit 5a55ddc
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diff --git a/narwhals/dtypes.py b/narwhals/dtypes.py
@@ -70,6 +70,14 @@ class Date(TemporalType): ...
 
 
 def translate_dtype(plx: Any, dtype: DType) -> Any:
+    if "polars" in str(type(dtype)):
+        msg = (
+            f"Expected Narwhals object, got: {type(dtype)}.\n\n"
+            "Perhaps you:\n"
+            "- Forgot a `nw.from_native` somewhere?\n"
+            "- Used `pl.Int64` instead of `nw.Int64`?"
+        )
+        raise TypeError(msg)
     if dtype == Float64:
         return plx.Float64
     if dtype == Float32:

diff --git a/tests/test_common.py b/tests/test_common.py
@@ -604,6 +604,8 @@ def test_invalid() -> None:
         df.select(nw.all() + nw.all())
     with pytest.raises(TypeError, match="Perhaps you:"):
         df.select([pl.col("a")])  # type: ignore[list-item]
+    with pytest.raises(TypeError, match="Perhaps you:"):
+        df.select([nw.col("a").cast(pl.Int64)])
 
 
 @pytest.mark.parametrize("df_raw", [df_pandas])