fix(python): Raise ValueError on passing multiple expressions Numpy u…

…func (#6821)

py-polars/polars/internals/expr/expr.py

@@ -290,6 +290,12 @@ def __array_ufunc__(
     ) -> Expr:
         """Numpy universal functions."""
 
+        num_expr = sum(isinstance(inp, Expr) for inp in inputs)
+        if num_expr > 1:
+            raise ValueError(
+                f"Numpy ufunc can only be used with one expression, {num_expr} given. Use `pl.reduce` to call numpy functions over multiple expressions."
+            )
+
         def function(s: pli.Series) -> pli.Series:  # pragma: no cover
             args = [inp if not isinstance(inp, Expr) else s for inp in inputs]
             return ufunc(*args, **kwargs)

py-polars/tests/unit/test_lazy.py

@@ -973,6 +973,20 @@ def test_ufunc_expr_not_first() -> None:
     assert_frame_equal(out, expected)
 
 
+def test_ufunc_multiple_expr() -> None:
+    df = pl.DataFrame(
+        [
+            pl.Series("a", [1, 2, 3], dtype=pl.Float64),
+            pl.Series("b", [4, 5, 6], dtype=pl.Float64),
+        ]
+    )
+
+    with pytest.raises(
+        ValueError, match="Numpy ufunc can only be used with one expression, 2 given"
+    ):
+        df.select(np.arctan2(pl.col("a"), pl.col("b")))  # type: ignore[call-overload]
+
+
 def test_clip() -> None:
     df = pl.DataFrame({"a": [1, 2, 3, 4, 5]})
     assert df.select(pl.col("a").clip(2, 4))["a"].to_list() == [2, 2, 3, 4, 4]