Hi all,
I'm encountering exceptions in a number of popular third-party Django
libraries when upgrading from 1.8 to 1.11. The code path is typically
`MyModel.objects.get_or_create(...)`, which causes
`model._meta._property_names` to be checked (to make sure we're not
querying/setting a property). The problem is, `_property_names` is
implemented as follows:
# in django/db/models/options.py:
def _property_names(self):
return frozenset({
attr for attr in
dir(self.model) if isinstance(getattr(self.model, attr), property)
})
The problem is when a custom field installs a field descriptor on the model
class (during `contribute_to_class()`), with a `__get__()` method like this:
class SomeFieldDescriptor(object):
# ...
def __get__(self, instance, type=None):
if instance is None:
raise AttributeError("Can only be accessed via an instance.")
# ...
Libraries which install such descriptors include
[django-fsm](https://github.com/kmmbvnr/django-fsm/blob/2d2eaee/django_fsm/__init__.py#L225)
and
[django-jsonfield](https://github.com/dmkoch/django-jsonfield/blob/2.0.1/jsonfield/subclassing.py#L35).
I think the problem is that we can't assume that all results of
`dir(model_class)` can be accessed via `getattr(model_class, attr)` without
exception; indeed, the Python docs state (c.f.
https://docs.python.org/2/library/functions.html#dir):
> Because dir() is supplied primarily as a convenience for use at an
interactive prompt, it tries to supply an interesting set of names more
than it tries to supply a rigorously or consistently defined set of names,
and its detailed behavior may change across releases.
A potential fix would be to adjust the definition of `_property_names` like
so:
def _property_names(self):
attrs = []
for attr in dir(self.model):
try:
if isinstance(getattr(self.model, attr), property):
attrs.append(attr)
except AttributeError:
pass
return frozenset(attrs)
Does this seem like a good solution, or even a problem worth solving?
Thanks!
-- Zack
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