Allow initializing Distance from angle-like parallax

astropy/coordinates/distances.py

@@ -50,7 +50,7 @@ class Distance(u.SpecificTypeQuantity):
     distmod : float or `~astropy.units.Quantity`
         The distance modulus for this distance. Note that if ``unit`` is not
         provided, a guess will be made at the unit between AU, pc, kpc, and Mpc.
-    parallax : `~astropy.units.Quantity` or `~astropy.coordinates.Angle`
+    parallax : angle-like
         The parallax in angular units.
     dtype : `~numpy.dtype`, optional
         See `~astropy.units.Quantity`.
@@ -162,6 +162,7 @@ def __new__(
                     unit = u.pc
 
         elif parallax is not None:
+            parallax = u.Quantity(parallax, copy=False)
             if unit is None:
                 unit = u.pc
             value = parallax.to_value(unit, equivalencies=u.parallax())

astropy/coordinates/tests/test_distance.py

@@ -10,6 +10,7 @@
 from astropy import units as u
 from astropy.coordinates import CartesianRepresentation, Distance, Latitude, Longitude
 from astropy.coordinates.builtin_frames import ICRS, Galactic
+from astropy.table import Column
 from astropy.units import allclose as quantity_allclose
 from astropy.utils.compat.optional_deps import HAS_SCIPY
 from astropy.utils.exceptions import AstropyWarning
@@ -313,3 +314,20 @@ def test_distance_to_quantity_when_not_units_of_length():
 def test_distance_nan():
     # Check that giving NaNs to Distance doesn't emit a warning
     Distance([0, np.nan, 1] * u.m)
+
+
+def test_distance_parallax_angle_like():
+    """Test angle-like behavior of Distance.parallax for object with a unit attribute.
+
+    Adapted from #15693
+    """
+    parallax = Column([1.0, 2.0, 3.0], unit=u.mas)
+    d0 = Distance(parallax=parallax)
+    d1 = Distance(parallax=u.Quantity(parallax))
+    assert np.all(d0 == d1)
+
+    class FloatMas(float):
+        unit = u.mas
+
+    d2 = Distance(parallax=FloatMas(1.0))
+    assert d0[0] == d2

docs/changes/coordinates/15712.bugfix.rst

@@ -0,0 +1,3 @@
+Fix a bug in the coordinates ``Distance`` class to allow initialization with
+``parallax`` as any angle-like value. This includes types like ``Column`` which have a
+unit but are not ``Quantity`` objects.