Made model fields comparable to other objects

Fixed #17851 -- Added __lt__ and @total_ordering to models.Field,
made sure these work correctly on other objects than Field, too.

django/db/models/fields/__init__.py

@@ -12,7 +12,7 @@ from django import forms
 from django.core import exceptions, validators
 from django.utils.datastructures import DictWrapper
 from django.utils.dateparse import parse_date, parse_datetime, parse_time
-from django.utils.functional import curry
+from django.utils.functional import curry, total_ordering
 from django.utils.text import capfirst
 from django.utils import timezone
 from django.utils.translation import ugettext_lazy as _
@@ -45,6 +45,7 @@ class FieldDoesNotExist(Exception):
 #
 #     getattr(obj, opts.pk.attname)
 
+@total_ordering
 class Field(object):
     """Base class for all field types"""
 
@@ -118,9 +119,17 @@ class Field(object):
         messages.update(error_messages or {})
         self.error_messages = messages
 
-    def __cmp__(self, other):
+    def __eq__(self, other):
+        # Needed for @total_ordering
+        if isinstance(other, Field):
+            return self.creation_counter == other.creation_counter
+        return NotImplemented
+
+    def __lt__(self, other):
         # This is needed because bisect does not take a comparison function.
-        return cmp(self.creation_counter, other.creation_counter)
+        if isinstance(other, Field):
+            return self.creation_counter < other.creation_counter
+        return NotImplemented
 
     def __deepcopy__(self, memodict):
         # We don't have to deepcopy very much here, since most things are not

django/utils/functional.py

@@ -310,3 +310,35 @@ def partition(predicate, values):
     for item in values:
         results[predicate(item)].append(item)
     return results
+
+try:
+    from functools import total_ordering
+except ImportError:
+    # For Python < 2.7
+    # Code borrowed from python 2.7.3 stdlib
+    def total_ordering(cls):
+        """Class decorator that fills in missing ordering methods"""
+        convert = {
+            '__lt__': [('__gt__', lambda self, other: not (self < other or self == other)),
+                       ('__le__', lambda self, other: self < other or self == other),
+                       ('__ge__', lambda self, other: not self < other)],
+            '__le__': [('__ge__', lambda self, other: not self <= other or self == other),
+                       ('__lt__', lambda self, other: self <= other and not self == other),
+                       ('__gt__', lambda self, other: not self <= other)],
+            '__gt__': [('__lt__', lambda self, other: not (self > other or self == other)),
+                       ('__ge__', lambda self, other: self > other or self == other),
+                       ('__le__', lambda self, other: not self > other)],
+            '__ge__': [('__le__', lambda self, other: (not self >= other) or self == other),
+                       ('__gt__', lambda self, other: self >= other and not self == other),
+                       ('__lt__', lambda self, other: not self >= other)]
+        }
+        roots = set(dir(cls)) & set(convert)
+        if not roots:
+            raise ValueError('must define at least one ordering operation: < > <= >=')
+        root = max(roots)       # prefer __lt__ to __le__ to __gt__ to __ge__
+        for opname, opfunc in convert[root]:
+            if opname not in roots:
+                opfunc.__name__ = opname
+                opfunc.__doc__ = getattr(int, opname).__doc__
+                setattr(cls, opname, opfunc)
+        return cls

tests/modeltests/basic/tests.py

@@ -3,7 +3,7 @@ from __future__ import absolute_import
 from datetime import datetime
 
 from django.core.exceptions import ObjectDoesNotExist
-from django.db.models.fields import FieldDoesNotExist
+from django.db.models.fields import Field, FieldDoesNotExist
 from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
 from django.utils.translation import ugettext_lazy
 
@@ -520,6 +520,22 @@ class ModelTest(TestCase):
         s = set([a10, a11, a12])
         self.assertTrue(Article.objects.get(headline='Article 11') in s)
 
+    def test_field_ordering(self):
+        """
+        Field instances have a `__lt__` comparison function to define an
+        ordering based on their creation. Prior to #17851 this ordering
+        comparison relied on the now unsupported `__cmp__` and was assuming
+        compared objects were both Field instances raising `AttributeError`
+        when it should have returned `NotImplemented`.
+        """
+        f1 = Field()
+        f2 = Field(auto_created=True)
+        f3 = Field()
+        self.assertTrue(f2 < f1)
+        self.assertTrue(f3 > f1)
+        self.assertFalse(f1 == None)
+        self.assertFalse(f2 in (None, 1, ''))
+
     def test_extra_method_select_argument_with_dashes_and_values(self):
         # The 'select' argument to extra() supports names with dashes in
         # them, as long as you use values().