Don't return Quantities from high_level_objects_to_values

astropy/wcs/wcsapi/high_level_api.py

@@ -1,4 +1,5 @@
 import abc
+import numbers
 from collections import OrderedDict, defaultdict
 
 import numpy as np
@@ -124,7 +125,8 @@
 
     This function uses the information in ``wcs.world_axis_object_classes`` and
     ``wcs.world_axis_object_components`` to convert the high level objects
-    (such as `~.SkyCoord`) to low level "values" `~.Quantity` objects.
+    (such as `~.SkyCoord`) to low level "values" which should be scalars or
+    Numpy arrays.
 
     This is used in `.HighLevelWCSMixin.world_to_pixel`, but provided as a
     separate function for use in other places where needed.
@@ -240,6 +242,17 @@
         else:
             world.append(rec_getattr(objects[key], attr))
 
+    # Check the type of the return values - should be scalars or plain Numpy
+    # arrays, not e.g. Quantity. Note that we deliberately use type(w) because
+    # we don't want to match Numpy subclasses.
+    for w in world:
+        if not isinstance(w, numbers.Number) and not type(w) == np.ndarray:
+            raise TypeError(
+                f"WCS world_axis_object_components results in "
+                f"values which are not scalars or plain Numpy "
+                f"arrays (got {type(w)})"
+            )
+
     return world
 
 
@@ -262,6 +275,16 @@
     low_level_wcs: `.BaseLowLevelWCS`
         The WCS object to use to interpret the coordinates.
     """
+    # Check the type of the input values - should be scalars or plain Numpy
+    # arrays, not e.g. Quantity. Note that we deliberately use type(w) because
+    # we don't want to match Numpy subclasses.
+    for w in world_values:
+        if not isinstance(w, numbers.Number) and not type(w) == np.ndarray:
+            raise TypeError(
+                f"Expected world coordinates as scalars or plain Numpy "
+                f"arrays (got {type(w)})"
+            )
+
     # Cache the classes and components since this may be expensive
     components = low_level_wcs.world_axis_object_components
     classes = low_level_wcs.world_axis_object_classes

astropy/wcs/wcsapi/tests/test_high_level_api.py

@@ -1,6 +1,10 @@
+import re
+
 import numpy as np
+import pytest
 from numpy.testing import assert_allclose
 
+from astropy import units as u
 from astropy.coordinates import SkyCoord
 from astropy.units import Quantity
 from astropy.wcs import WCS
@@ -205,6 +209,55 @@ def test_values_to_objects():
     assert c2.b == c2_out.b
 
 
+class InvalidWCSQuantity(SkyCoordDuplicateWCS):
+    """
+    WCS which defines ``world_axis_object_components`` which returns Quantity
+    instead of bare Numpy arrays, which can cause issues. This is for a
+    regression test to make sure that we don't return Quantities from
+    ``world_axis_object_components``.
+    """
+
+    @property
+    def world_axis_object_components(self):
+        return [
+            ("test1", "ra", "spherical.lon"),
+            ("test1", "dec", "spherical.lat"),
+            ("test2", 0, "spherical.lon"),
+            ("test2", 1, "spherical.lat"),
+        ]
+
+
+def test_objects_to_values_invalid_type():
+    wcs = InvalidWCSQuantity()
+    c1, c2 = wcs.pixel_to_world(1, 2, 3, 4)
+    with pytest.raises(
+        TypeError,
+        match=(
+            re.escape(
+                "WCS world_axis_object_components results in values which are not "
+                "scalars or plain Numpy arrays (got <class "
+                "'astropy.coordinates.angles.core.Longitude'>)"
+            )
+        ),
+    ):
+        high_level_objects_to_values(c1, c2, low_level_wcs=wcs)
+
+
+def test_values_to_objects_invalid_type():
+    wcs = SkyCoordDuplicateWCS()
+    c1, c2 = wcs.pixel_to_world(1, 2, 3, 4)
+    with pytest.raises(
+        TypeError,
+        match=(
+            re.escape(
+                "Expected world coordinates as scalars or plain Numpy arrays (got "
+                "<class 'astropy.units.quantity.Quantity'>)"
+            )
+        ),
+    ):
+        values_to_high_level_objects(2 * u.m, 4, 6, 8, low_level_wcs=wcs)
+
+
 class MinimalHighLevelWCS(HighLevelWCSMixin):
     def __init__(self, low_level_wcs):
         self._low_level_wcs = low_level_wcs

docs/changes/wcs/16287.api.rst

@@ -0,0 +1,3 @@
+Errors may now occur if a ``BaseLowLevelWCS`` class defines
+``world_axis_object_components`` which returns values that are not scalars or
+plain Numpy arrays as per APE 14.