Table's subclass break with serialization

[saimn]

With the serialization of astropy objects to FITS (#6912), I just noticed that it breaks the use of our Table subclass. When writing and reading back a FITS file with this subclass, io.registry.read now fails because the reader return a QTable, for which my class is not a subclass.
I don't see a way to disable the serialization, or to not use it while reading. Any idea @taldcroft ?

[saimn]

Actually the failure is because of #7147, and because we set .format and .description on our columns (to save both as FITS and VOTable).

[pllim]

(Ops, I kept adding a milestone by habit... I removed it. Ignore the confusing history there.)

[taldcroft]

Here is a demonstration of the problem:

from astropy.table import Table
import astropy.units as u

class MTable(Table):
    pass

mt = MTable([[1, 2.5]], names=['a'])
mt['a'].unit = u.m
mt['a'].format = '.4f'
mt['a'].description = 'hello'

mt.write('junk.fits', overwrite=True)
mt2 = MTable.read('junk.fits')

Here is a potential fix. Basically it just punts on making sure that the object the reader returns is a strict subclass of the output class, and instead just tries converting. This works for Table/QTable. Not sure if removing this test is a worry for other reader classes. @astrofrog ?

(astropy) neptune$ git diff
diff --git a/astropy/io/registry.py b/astropy/io/registry.py
index b332f83..d9df055 100644
--- a/astropy/io/registry.py
+++ b/astropy/io/registry.py
@@ -517,18 +517,15 @@ def read(cls, *args, format=None, **kwargs):
         data = reader(*args, **kwargs)
 
         if not isinstance(data, cls):
-            if issubclass(cls, data.__class__):
-                # User has read with a subclass where only the parent class is
-                # registered.  This returns the parent class, so try coercing
-                # to desired subclass.
-                try:
-                    data = cls(data)
-                except Exception:
-                    raise TypeError('could not convert reader output to {0} '
-                                    'class.'.format(cls.__name__))
-            else:
-                raise TypeError("reader should return a {0} instance"
-                                "".format(cls.__name__))
+            # User has read with a subclass where only the parent class is
+            # registered.  This returns the parent class, so try coercing
+            # to desired subclass.
+            try:
+                data = cls(data)
+            except Exception:
+                raise TypeError('could not convert reader output to {0} '
+                                'class.'.format(cls.__name__))
+

I'm supposed to be on holiday this week so if anyone else wants to pick this up that would be good. 😄

[taldcroft]

BTW see line below for the source of the QTable, and see #7190 for discussion related to why it needs to be QTable at that point:

astropy/astropy/table/serialize.py

Line 199 in 9878f93

# Don't know final output class but assume QTable so no columns get

[saimn]

Thanks for the quick answer and the patch @taldcroft , I can have a look to integrate this with a test if you want to enjoy your holidays 😉