[1.7.x] Fixed #23112 -- Field.get_choices tries to index an iterable

Backport of 97a38de230 from master.

django/db/models/fields/__init__.py

@@ -730,9 +730,10 @@ class Field(RegisterLookupMixin):
         """Returns choices with a default blank choices included, for use
         as SelectField choices for this field."""
         blank_defined = False
-        named_groups = self.choices and isinstance(self.choices[0][1], (list, tuple))
+        choices = list(self.choices) if self.choices else []
+        named_groups = choices and isinstance(choices[0][1], (list, tuple))
         if not named_groups:
-            for choice, __ in self.choices:
+            for choice, __ in choices:
                 if choice in ('', None):
                     blank_defined = True
                     break
@@ -740,7 +741,7 @@ class Field(RegisterLookupMixin):
         first_choice = (blank_choice if include_blank and
                         not blank_defined else [])
         if self.choices:
-            return first_choice + list(self.choices)
+            return first_choice + choices
         rel_model = self.rel.to
         if hasattr(self.rel, 'get_related_field'):
             lst = [(getattr(x, self.rel.get_related_field().attname),

tests/model_fields/models.py

@@ -45,6 +45,29 @@ class Whiz(models.Model):
     c = models.IntegerField(choices=CHOICES, null=True)
 
 
+class Counter:
+    def __init__(self):
+        self.n = 1
+
+    def __iter__(self):
+        return self
+
+    def next(self):  # Python 3: def __next__(self)
+        if self.n > 5:
+            raise StopIteration
+        else:
+            self.n += 1
+            return (self.n, 'val-'+str(self.n))
+
+
+class WhizIter(models.Model):
+    c = models.IntegerField(choices=Counter(), null=True)
+
+
+class WhizIterEmpty(models.Model):
+    c = models.CharField(choices=(x for x in []), blank=True, max_length=1)
+
+
 class BigD(models.Model):
     d = models.DecimalField(max_digits=38, decimal_places=30)
 

tests/model_fields/tests.py

@@ -25,7 +25,8 @@ from .models import (
     Foo, Bar, Whiz, BigD, BigS, BigIntegerModel, Post, NullBooleanModel,
     BooleanModel, PrimaryKeyCharModel, DataModel, Document, RenamedField,
     DateTimeModel, VerboseNameField, FksToBooleans, FkToChar, FloatModel,
-    SmallIntegerModel, IntegerModel, PositiveSmallIntegerModel, PositiveIntegerModel)
+    SmallIntegerModel, IntegerModel, PositiveSmallIntegerModel, PositiveIntegerModel,
+    WhizIter, WhizIterEmpty)
 
 
 class BasicFieldTests(test.TestCase):
@@ -375,6 +376,31 @@ class ChoicesTests(test.TestCase):
         self.assertEqual(Whiz(c=None).get_c_display(), None)    # Blank value
         self.assertEqual(Whiz(c='').get_c_display(), '')        # Empty value
 
+    def test_iterator_choices(self):
+        """
+        Check that get_choices works with Iterators (#23112).
+        """
+        self.assertEqual(WhizIter(c=1).c, 1)          # A nested value
+        self.assertEqual(WhizIter(c=9).c, 9)          # Invalid value
+        self.assertEqual(WhizIter(c=None).c, None)    # Blank value
+        self.assertEqual(WhizIter(c='').c, '')        # Empty value
+
+    def test_empty_iterator_choices(self):
+        """
+        Check that get_choices works with empty iterators (#23112).
+        """
+        self.assertEqual(WhizIterEmpty(c="a").c, "a")      # A nested value
+        self.assertEqual(WhizIterEmpty(c="b").c, "b")      # Invalid value
+        self.assertEqual(WhizIterEmpty(c=None).c, None)    # Blank value
+        self.assertEqual(WhizIterEmpty(c='').c, '')        # Empty value
+
+    def test_charfield_get_choices_with_blank_iterator(self):
+        """
+        Check that get_choices works with an empty Iterator
+        """
+        f = models.CharField(choices=(x for x in []))
+        self.assertEqual(f.get_choices(include_blank=True), [('', '---------')])
+
 
 class SlugFieldTests(test.TestCase):
     def test_slugfield_max_length(self):