SubFieldBase descriptor object should be accessible

[Lars van de Kerkhof]

Currently any field that has SubFieldBase as a metaclass will throw an exception if you try access the descriptor object without an instance.
Things like the following can not be done with the current implementation:

    def _get_members_of_type(obj, member_type):
        """
        Finds members of a certain type in obj.
        """
        def filter_member_type(member):
            try:
                return type(member) is member_type
            except AttributeError:
                return False
                
        # if the type of obj is the metaclass for all models, just search in the object
        # because it is not a model instance but a type
        if type(obj) is ModelBase:
            key_hash = inspect.getmembers(obj, filter_member_type)
        else:
            key_hash = inspect.getmembers(obj.__class__, filter_member_type)
            

        return key_hash

SubFieldBase will throw an AttributeError every time inspect.getmembers is used on a model instance or class.
Normal behaviour would be to return the descriptor object when no instance is passed. All django's standard fields
behave this way, so it would be best if SubFieldBase would do the same.

I will attach a patch against current trunk.

[Lars van de Kerkhof]

More specific:

SubFieldBase? will throw an AttributeError? every time inspect.getmembers is used on a model instance or class that has fields which use SubFieldBase.

[James Bennett]

1.2 is feature-frozen, moving this feature request off the milestone.

[Lars van de Kerkhof]

I added a new patch + tests to fix this issue in 1.2
Please do not let descriptors break inspect, but instead behave like a proper property, which returns self when a descriptor is accessed without context:

​http://users.rcn.com/python/download/Descriptor.htm#properties

If you need the descriptor object itself to be inaccessible for some reason, just return None instead of self. This would also make more sense, an uninitialized property is None, just like a regular attribute.

There are more places where the AttributeError is thrown, for instance ManagerDescriptor raises:

AttributeError: Manager isn't accessible via MyModel instances

When a model instance is inspected.

ImageField and FileField also do it, which to me seems wrong.

[Lars van de Kerkhof]

This time the test is also added

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[c v t]

Rebased against git master (5be56d0)

[c v t]

Any chance this can make it into an official release? I'm happy to polish up however needed, or take this to django-developers if it really needs discussion.

Rebased patch against git master attached above, as a start.

This bug causes an AttributeError when trying to use sphinx-autodoc on custom fields which have __metaclass__ = SubFieldBase, due to the underlying "must be accessed from instance" AttributeError. The equivalent ticket for fixing inspect() for Django's built-in fields was #8248, fixed in [9634] (~5 years ago).

[anonymous]

Nice!

[Anssi Kääriäinen]

Any objections for adding this, seems like a good addition to me?

[Anssi Kääriäinen <akaariai@…>]

In 50633e7353694ff54f14b04469be3792f286182f:

Fixed #12568 -- no error when accessing custom field's descriptor

The SubfieldBase's descriptor caused an AttributeError when accessed
from the class. Introspection didn't like that.

Patch by Trac alias supervacuo.