Add constant PacketNodeOrder

Author: felixblanke

src/ptwt/constants.py

@@ -51,6 +51,14 @@
 Choose ``gramschmidt`` if ``qr`` runs out of memory.
 """
 
+PacketNodeOrder = Literal["freq", "natural"]
+"""
+This is a type literal for the order of wavelet packet tree nodes.
+
+- frequency order (``freq``)
+- natural order (``natural``)
+"""
+
 
 class WaveletDetailTuple2d(NamedTuple):
     """Detail coefficients of a 2d wavelet transform for a given level.

src/ptwt/packets.py

@@ -16,6 +16,7 @@
 from .constants import (
     ExtendedBoundaryMode,
     OrthogonalizeMethod,
+    PacketNodeOrder,
     WaveletCoeff2d,
     WaveletCoeffNd,
     WaveletDetailTuple2d,
@@ -205,7 +206,7 @@ def _get_waverec(
             return partial(waverec, wavelet=self.wavelet, axis=self.axis)
 
     @staticmethod
-    def get_level(level: int, order: Literal["freq", "natural"] = "freq") -> list[str]:
+    def get_level(level: int, order: PacketNodeOrder = "freq") -> list[str]:
         """Return the paths to the filter tree nodes.
 
         Args:
@@ -540,7 +541,7 @@ def get_level(level: int, order: Literal["natural"]) -> list[str]: ...
 
     @staticmethod
     def get_level(
-        level: int, order: Literal["freq", "natural"] = "freq"
+        level: int, order: PacketNodeOrder = "freq"
     ) -> Union[list[str], list[list[str]]]:
         """Return the paths to the filter tree nodes.