[1.5.x] Fixed #20094 - Be more careful when checking for Iterator

Python 2.6 has some different behaviour when checking
isinstance(foo, collections.Iterator).
Backport of 829dc3c5 from master.

django/db/models/fields/__init__.py

@@ -1,6 +1,5 @@
 from __future__ import unicode_literals
 
-import collections
 import copy
 import datetime
 import decimal
@@ -16,6 +15,7 @@ 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, total_ordering
+from django.utils.itercompat import is_iterator
 from django.utils.text import capfirst
 from django.utils import timezone
 from django.utils.translation import ugettext_lazy as _
@@ -444,7 +444,7 @@ class Field(object):
         return bound_field_class(self, fieldmapping, original)
 
     def _get_choices(self):
-        if isinstance(self._choices, collections.Iterator):
+        if is_iterator(self._choices):
             choices, self._choices = tee(self._choices)
             return choices
         else:

django/db/models/sql/where.py

@@ -4,7 +4,6 @@ Code to manage the creation and SQL rendering of 'where' constraints.
 
 from __future__ import absolute_import
 
-import collections
 import datetime
 from itertools import repeat
 
@@ -12,6 +11,7 @@ from django.utils import tree
 from django.db.models.fields import Field
 from django.db.models.sql.datastructures import EmptyResultSet
 from django.db.models.sql.aggregates import Aggregate
+from django.utils.itercompat import is_iterator
 from django.utils.six.moves import xrange
 
 # Connection types
@@ -51,7 +51,7 @@ class WhereNode(tree.Node):
             return
 
         obj, lookup_type, value = data
-        if isinstance(value, collections.Iterator):
+        if is_iterator(value):
             # Consume any generators immediately, so that we can determine
             # emptiness and transform any non-empty values correctly.
             value = list(value)

django/utils/itercompat.py

@@ -4,10 +4,12 @@ Where possible, we try to use the system-native version and only fall back to
 these implementations if necessary.
 """
 
-from django.utils.six.moves import builtins
+import collections
 import itertools
+import sys
 import warnings
 
+
 def is_iterable(x):
     "A implementation independent way of checking for iterables"
     try:
@@ -17,6 +19,17 @@ def is_iterable(x):
     else:
         return True
 
+def is_iterator(x):
+    """An implementation independent way of checking for iterators
+
+    Python 2.6 has a different implementation of collections.Iterator which
+    accepts anything with a `next` method. 2.7+ requires and `__iter__` method
+    as well.
+    """
+    if sys.version_info >= (2, 7):
+        return isinstance(x, collections.Iterator)
+    return isinstance(x, collections.Iterator) and hasattr(x, '__iter__')
+
 def product(*args, **kwds):
     warnings.warn("django.utils.itercompat.product is deprecated; use the native version instead",
                   PendingDeprecationWarning)

tests/regressiontests/utils/itercompat.py

@@ -0,0 +1,11 @@
+from django.test import TestCase
+
+from .models import Category, Thing
+
+
+class TestIsIterator(TestCase):
+    def test_regression(self):
+        """This failed on Django 1.5/Py2.6 because category has a next method."""
+        category = Category.objects.create(name='category')
+        Thing.objects.create(category=category)
+        Thing.objects.filter(category=category)

tests/regressiontests/utils/models.py

@@ -1 +1,13 @@
-# Test runner needs a models.py file.
+from django.db import models
+
+
+class Category(models.Model):
+    name = models.CharField(max_length=100)
+
+    def next(self):
+        return self
+
+
+class Thing(models.Model):
+    name = models.CharField(max_length=100)
+    category = models.ForeignKey(Category)

tests/regressiontests/utils/tests.py

@@ -19,6 +19,7 @@ from .functional import FunctionalTestCase
 from .html import TestUtilsHtml
 from .http import TestUtilsHttp, ETagProcessingTests, HttpDateProcessingTests
 from .ipv6 import TestUtilsIPv6
+from .itercompat import TestIsIterator
 from .jslex import JsToCForGettextTest, JsTokensTest
 from .module_loading import CustomLoader, DefaultLoader, EggLoader
 from .numberformat import TestNumberFormat