Ensure that quantities are converted to Magnitude instances if needed.

astropy/units/quantity.py

@@ -594,13 +594,15 @@ def __quantity_subclass__(self, unit):
 
     def _new_view(self, obj=None, unit=None):
         """
-        Create a Quantity view of obj, and set the unit
+        Create a Quantity view of some array-like input, and set the unit
 
-        By default, return a view of ``obj`` of the same class as ``self``
-        and with the unit passed on, or that of ``self``.  Subclasses can
-        override the type of class used with ``__quantity_subclass__``, and
-        can ensure other properties of ``self`` are copied using
-        `__array_finalize__`.
+        By default, return a view of ``obj`` of the same class as ``self`` and
+        with the same unit.  Subclasses can override the type of class for a
+        given unit using ``__quantity_subclass__``, and can ensure properties
+        other than the unit are copied using ``__array_finalize__``.
+
+        If the given unit defines a ``_quantity_class`` of which ``self``
+        is not an instance, a view using this class is taken.
 
         Parameters
         ----------
@@ -624,9 +626,11 @@ def _new_view(self, obj=None, unit=None):
             quantity_subclass = self.__class__
         else:
             unit = Unit(unit)
-            quantity_subclass, subok = self.__quantity_subclass__(unit)
-            if subok:
-                quantity_subclass = self.__class__
+            quantity_subclass = getattr(unit, '_quantity_class', Quantity)
+            if isinstance(self, quantity_subclass):
+                quantity_subclass, subok = self.__quantity_subclass__(unit)
+                if subok:
+                    quantity_subclass = self.__class__
 
         # We only want to propagate information from ``self`` to our new view,
         # so obj should be a regular array.  By using ``np.array``, we also

astropy/units/tests/test_logarithmic.py

@@ -377,6 +377,23 @@ def test_indirect_creation(self, unit):
         assert (q2._function_view / u.mag).to(1).value == -5.
 
 
+def test_conversion_to_and_from_physical_quantities():
+    """Ensures we can convert from regular quantities."""
+    mst = [10., 12., 14.] * u.STmag
+    flux_lambda = mst.physical
+    mst_roundtrip = flux_lambda.to(u.STmag)
+    # check we return a logquantity; see #5178.
+    assert isinstance(mst_roundtrip, u.Magnitude)
+    assert mst_roundtrip.unit == mst.unit
+    assert_allclose(mst_roundtrip.value, mst.value)
+    wave = [4956.8, 4959.55, 4962.3] * u.AA
+    flux_nu = mst.to(u.Jy, equivalencies=u.spectral_density(wave))
+    mst_roundtrip2 = flux_nu.to(u.STmag, u.spectral_density(wave))
+    assert isinstance(mst_roundtrip2, u.Magnitude)
+    assert mst_roundtrip2.unit == mst.unit
+    assert_allclose(mst_roundtrip2.value, mst.value)
+
+
 class TestLogQuantityViews(object):
     def setup(self):
         self.lq = u.Magnitude(np.arange(10.) * u.Jy)