Check values equal to zero in QuantityAttribute a bit more carefully.

astropy/coordinates/attributes.py

@@ -321,20 +321,24 @@ def convert_input(self, value):
         if value is None:
             return None, False
 
-        if np.all(value == 0) and self.unit is not None:
-            return u.Quantity(np.zeros(self.shape), self.unit), True
-        else:
-            if not hasattr(value, 'unit') and self.unit != u.dimensionless_unscaled:
-                raise TypeError('Tried to set a QuantityAttribute with '
-                                'something that does not have a unit.')
-            oldvalue = value
-            value = u.Quantity(oldvalue, self.unit, copy=False)
-            if self.shape is not None and value.shape != self.shape:
-                raise ValueError('The provided value has shape "{}", but '
-                                 'should have shape "{}"'.format(value.shape,
-                                                                 self.shape))
-            converted = oldvalue is not value
-            return value, converted
+        if (not hasattr(value, 'unit') and self.unit != u.dimensionless_unscaled
+                and np.any(value != 0)):
+            raise TypeError('Tried to set a QuantityAttribute with '
+                            'something that does not have a unit.')
+
+        oldvalue = value
+        value = u.Quantity(oldvalue, self.unit, copy=False)
+        if self.shape is not None and value.shape != self.shape:
+            if value.shape == () and oldvalue == 0:
+                # Allow a single 0 to fill whatever shape is needed.
+                value = np.broadcast_to(value, self.shape, subok=True)
+            else:
+                raise ValueError(
+                    f'The provided value has shape "{value.shape}", but '
+                    f'should have shape "{self.shape}"')
+
+        converted = oldvalue is not value
+        return value, converted
 
 
 class EarthLocationAttribute(Attribute):

astropy/coordinates/tests/test_frames.py

@@ -1018,6 +1018,38 @@ class MyCoord2(BaseCoordinateFrame):
     frame = MyCoord2()
     assert u.isclose(frame.someval, 15*u.deg)
 
+    # Since here no shape was given, we can set to any shape we like.
+    frame = MyCoord2(someval=np.ones(3)*u.deg)
+    assert frame.someval.shape == (3,)
+    assert np.all(frame.someval == 1*u.deg)
+
+    # We should also be able to insist on a given shape.
+    class MyCoord3(BaseCoordinateFrame):
+        someval = QuantityAttribute(unit=u.arcsec, shape=(3,))
+
+    frame = MyCoord3(someval=np.ones(3)*u.deg)
+    assert frame.someval.shape == (3,)
+    assert frame.someval.unit == u.arcsec
+    assert u.allclose(frame.someval.value, 3600.)
+
+    # The wrong shape raises.
+    with pytest.raises(ValueError, match='shape'):
+        MyCoord3(someval=1.*u.deg)
+
+    # As does the wrong unit.
+    with pytest.raises(u.UnitsError):
+        MyCoord3(someval=np.ones(3)*u.m)
+
+    # We are allowed a short-cut for zero.
+    frame0 = MyCoord3(someval=0)
+    assert frame0.someval.shape == (3,)
+    assert frame0.someval.unit == u.arcsec
+    assert np.all(frame0.someval.value == 0.)
+
+    # But not if it has the wrong shape.
+    with pytest.raises(ValueError, match='shape'):
+        MyCoord3(someval=np.zeros(2))
+
     # This should fail, if we don't pass in a default or a unit
     with pytest.raises(ValueError):
         class MyCoord(BaseCoordinateFrame):