use is_scalar

Author: MarcoGorelli

pandas/core/col.py

@@ -9,6 +9,8 @@
     Any,
 )
 
+from pandas.core.dtypes.common import is_scalar
+
 from pandas.core.series import Series
 
 if TYPE_CHECKING:
@@ -32,9 +34,9 @@ def __init__(self, func: Callable[[DataFrame], Any]) -> None:
 
     def __call__(self, df: DataFrame) -> Series:
         result = self._func(df)
-        if not isinstance(result, Series):
+        if not (isinstance(result, Series) or is_scalar(result)):
             msg = (
-                "Expected function which returns Series, "
+                "Expected function which returns Series or scalar, "
                 f"got function which returns: {type(result)}"
             )
             raise TypeError(msg)