Add warning about spectral_axis_index being ignored in 1.x (#1238)

* Add warning about spectral_axis_index being ignored in 1.x

* Slight change

* Fix failing test

* Ignore UnitBase warning in readthedocs

* Update changelog

Author: rosteen

CHANGES.rst

@@ -1,15 +1,3 @@
-1.21.0 (unreleased)
--------------------
-
-New Features
-^^^^^^^^^^^^
-
-Bug Fixes
-^^^^^^^^^
-
-Other Changes and Additions
-^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 1.20.2 (unreleased)
 -------------------
 
@@ -22,6 +10,8 @@ Bug Fixes
 Other Changes and Additions
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+- Warn about use of ``spectral_axis_index`` (new Spectrum1D kwarg for 2.0) in 1.x. [#1238]
+
 1.20.1 (2025-05-07)
 -------------------
 

docs/nitpick-exceptions

@@ -7,6 +7,7 @@ py:obj NDUncertainty
 
 # Links that might not work yet, depending on the astropy version
 py:obj astropy.coordinates.SpectralQuantity
+py:class astropy.units.UnitBase
 
 # Classes in the SpectralCoord/SpectralQuantity methods for which we can't
 # control the API links.

specutils/spectra/spectrum1d.py

@@ -74,15 +74,22 @@ def __init__(self, flux=None, spectral_axis=None, wcs=None,
                  radial_velocity=None, bin_specification=None, **kwargs):
 
         # These will be new keywords in 2.x, we pop them here in case the
-        # alias for the new class name is being used
-        if 'move_spectral_axis' in kwargs:
+        # alias for the new class name is being used, and warn in cases that
+        # would result in different behavior between 1.x and 2.x
+        if 'move_spectral_axis' in kwargs or 'spectral_axis_index' in kwargs:
             good_inds = ['last', -1, None]
-            if flux is not None:
+            if flux is not None and hasattr(flux, 'ndim'):
                 good_inds.append(flux.ndim - 1)
+
+        if 'move_spectral_axis' in kwargs:
             move_spectral_axis = kwargs.pop('move_spectral_axis', None)
             if move_spectral_axis not in good_inds:
-                warnings.warn("move_spectral_axis is ignored in specutils 1.x.")
-        kwargs.pop('spectral_axis_index', None)
+                warnings.warn('move_spectral_axis is ignored in specutils 1.x.')
+
+        if 'spectral_axis_index' in kwargs:
+            spectral_axis_index = kwargs.pop('spectral_axis_index', None)
+            if spectral_axis_index not in good_inds:
+                warnings.warn('spectral_axis_index is ignored in specutils 1.x.')
 
         # Check for pre-defined entries in the kwargs dictionary.
         unknown_kwargs = set(kwargs).difference(