#17851: Db Field instances cannot be to other objects
----------------------------------------------+---------------------------
Reporter: charettes | Owner: charettes
Type: Bug | Status: new
Component: Database layer (models, ORM) | Version: 1.3
Severity: Normal | Keywords: field compare
Triage Stage: Unreviewed | Has patch: 1
Easy pickings: 0 | UI/UX: 0
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[https://github.com/django/django/blob/master/django/db/models/fields/__init__.py#L126-L128
`django.db.models.fields.Field.__cmp__`] assumes that it's always compared
to a another `Field` instance thus raising an `AttributeError` when the
following code is executed:
{{{#!python
from django.db.models.fields import Field
Field() == None
}}}
It should make sure that `other` is a `Field` instance before accessing
the `creation_counter` property and if not return `NotImplemented`.
I stumbled on this issue when trying to use a `PickledObjectField` to
store `Field` instance. When calling `full_clean` on the field it raised
an `AttributeError` because it was trying to compare it `EMPTY_VALUES`:
{{{value in EMPTY_VALUES}}}.
I agree that this is NOT a common use case of the Field class however I
think that `Field` instances should be able to compare themselves to other
objects without raising a confusing `AttributeError`.
I'm attaching a patch that sticks with simple comparison (although we
should REALLY move to advanced comparison) with tests.
Here's a chat log I had on #python concerning simple comparison:
{{{
charettes 13:31:23
What should an object that implements __cmp__ method should return if it
can't compare to other.
papna 13:31:49
charettes: 1) Don't write __cmp__ methods unless you're targetting
mesozoic Python 2) NotImplemented
charettes 13:32:41
papna: But the doc doesn't say that... it says only to use it for advanced
comparaison
charettes 13:32:58
papna: http://docs.python.org/reference/datamodel.html#object.__cmp__
papna 13:33:42
charettes: Why are you using __cmp__?
charettes 13:34:42
papna: actually I'm writing a patch for a project that used this method...
should I suggest to switch to adavanced comparaison?
papna 13:36:43
charettes: IIRC __cmp__ still uses the paradigm that __eq__ and friends
and __add__ and friends and plenty of other stuff does, but I might be
wrong. It's not flexible, which is why it has been deprecated well over a
decade.
charettes 13:37:27
papna: aight ty for your time
}}}
I can adapt the current patch to use advanced comparison instead if you
prefer.
--
Ticket URL: <https://code.djangoproject.com/ticket/17851>
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