id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
28490	Descriptors on Models are reported as nonexistent by System Check Framework for ModelAdmin.list_display if they return None	Hunter Richards	nobody	"= Brief Description

The Django ModelAdmin System Checks will erroneously mark as invalid any reference in `list_display` to a Descriptor on that ModelAdmin's model that returns `None` when accessed on that model's class, even if the Descriptor would return non-null values in the resulting ListView.

This error is due to code that subtly conflates a reference to a field descriptor with the return value of that descriptor.  In this ticket, I give an in-depth explanation of the bug and a short fix, presented inline, for demonstration purposes, and a larger fix containing an exciting refactor and a justification for it.


= Example

Given a model with a field that is a Descriptor that returns `None`:

{{{#!python
class DescriptorThatReturnsNone(object):
    def __get__(self, obj, objtype):
        return None

    def __set__(self, obj, value):
        pass


class ValidationTestModel(models.Model):
    some_other_field = models.CharField(max_length=100)
    none_descriptor = DescriptorThatReturnsNone()
}}}

and a ModelAdmin that includes that Descriptor field in `list_display`:

{{{#!python
class TestModelAdmin(ModelAdmin):
    list_display = ('some_other_field', 'none_descriptor')
}}}

then the system checks that run at project start will fail to find that Descriptor attribute with the following error message:

{{{
(admin.E108) The value of 'list_display[4]' refers to 'none_descriptor', which is not a callable, an attribute of 'TestModelAdmin', or an attribute or method on 'modeladmin.ValidationTestModel'.
}}}


= Location of Error

The location of the error is in the following code:

https://github.com/django/django/blob/335aa088245a4cc6f1b04ba098458845344290bd/django/contrib/admin/checks.py#L586-L607

{{{#!python
        elif hasattr(model, item):
            # getattr(model, item) could be an X_RelatedObjectsDescriptor
            try:
                field = model._meta.get_field(item)
            except FieldDoesNotExist:
                try:
                    field = getattr(model, item)
                except AttributeError:
                    field = None

            if field is None:
                return [
                    checks.Error([...]
}}}

We follow the branch for `has_attr(model, item)`, and `field = model._meta.get_field(item)` throws an error, so we call `field = getattr(model, item)` for the purpose of either getting a reference to this non-field attribute or its value itself, depending on the type of `item`.  Supposedly, if `getattr` were to throw an error, we'd set `field` to none and return E108 (more on this below), but in the Descriptor case above, we don't.  But `field` is still `None`, i.e. the return value of the descriptor, so E108 is triggered anyway, even though the Descriptor field is a perfectly valid value to display in a ListView.

The cause of the error comes from confusing the ""type"" of `field`: when using Descriptors, it is not always the case that `getattr(SomeClass, some_attr_name)` will return a reference to the field in question and `getattr(some_class_instance, some_attr_name)` will return the actual value of the attribute, which is the unstated assumption of the quoted code.  Therefore, `field` sometimes references a field itself, and sometimes a field's value.


= A Workaround and a Quick Fix

I triggered this error with a slightly more complicated Descriptor that returned the value of a related field on its declared model, and `None` if that were not possible.  Realizing the special value `None` was at the heart of the error, as a workaround I rewrote the Descriptor to return the empty string as an ""empty"" value instead of `None` which is indistinguishable from `None` in a ListView.

As an example of how I intend to fix the error, here's The Simplest Thing that Could Possibly Work:

{{{#!diff
diff --git a/django/contrib/admin/checks.py b/django/contrib/admin/checks.py
index 830a190..b6b49a5 100644
--- a/django/contrib/admin/checks.py
+++ b/django/contrib/admin/checks.py
@@ -592,9 +592,11 @@ class ModelAdminChecks(BaseModelAdminChecks):
                 field = model._meta.get_field(item)
             except FieldDoesNotExist:
                 try:
-                    field = getattr(model, item)
+                    getattr(model, item)
                 except AttributeError:
                     field = None
+                else:
+                    field = True

             if field is None:
                 return [
}}}

In this fix, I discard the return value of `getattr` and just set `field` based on whether `getattr` succeeded or not.  The ""type"" of `field` is still a bit confused since `True` isn't a field type, but this fix won't miss any fields that happen to have the value `None` on classes.


= A Better Fix and a Refactor

But wait!  We've rightly discarded the return value of `getattr` above, but how'd we get to this line of code in the first place?  By having `hasattr(model, item)` be `True`.  According to the Python docs, `hasattr` is implemented using `getattr`:

https://docs.python.org/2/library/functions.html#hasattr
https://docs.python.org/3/library/functions.html#hasattr

> [`hasattr`] is implemented by calling `getattr(object, name)` and seeing whether it raises an exception or not.

So if we've reached the call to `getattr` above, then it follows by the definition of `hasattr` that we've already run `getattr` successfully.

If we continue this line of thinking, we realize that `getattr` thus cannot possibly fail in this branch, and in our new ""simple fix"" version above, `item` in this `elif` branch will never refer to an attribute that cannot be gotten through `getattr` for successful display in the ListView.  We can thus remove the first duplicated instance where E108 is raised, and thereby simplify the control flow greatly, so that the final version of this function resembles the following:

{{{#!python
    def _check_list_display_item(self, obj, model, item, label):
        if callable(item):
            return []
        elif hasattr(obj, item):
            return []
        elif hasattr(model, item):
            try:
                field = model._meta.get_field(item)
            except FieldDoesNotExist:
                return []
            else:
                if isinstance(field, models.ManyToManyField):
                    return [checks.Error([...E109: Disallowed M2Ms...])]
                else:
                    return []
        else:
            return [checks.Error([...E108...])]

}}}

which coincidentally matches the text of E108 much more closely.


= Submitting a Patch

I've attached a patch version of my changes to this PR for convenience, but I intend to open a Pull Request on GitHub that will contain all changes mentioned in this Ticket.  Both the patch and the PR will contain tests recapitulating and then demonstrating the fixing of this error."	Bug	closed	contrib.admin	dev	Normal	fixed	admin, descriptor, system, checks, framework, validation	Lucas Wiman	Accepted	1	0	0	0	0	0