TYP: improved _cubicspline_interpolate function annotations

Author: jorenham

pandas/core/missing.py

@@ -45,8 +45,12 @@
 )
 
 if TYPE_CHECKING:
+    from typing import TypeAlias
+
     from pandas import Index
 
+    _CubicBC: TypeAlias = Literal["not-a-knot", "clamped", "natural", "periodic"]
+
 
 def check_value_size(value, mask: npt.NDArray[np.bool_], length: int):
     """
@@ -700,9 +704,9 @@ def _cubicspline_interpolate(
     yi: np.ndarray,
     x: np.ndarray,
     axis: AxisInt = 0,
-    bc_type: str | tuple[Any, Any] = "not-a-knot",
-    extrapolate=None,
-):
+    bc_type: _CubicBC | tuple[Any, Any] = "not-a-knot",
+    extrapolate: Literal["periodic"] | bool | None = None,
+) -> np.ndarray:
     """
     Convenience function for cubic spline data interpolator.