Fixed #24483 -- Prevented keepdb from breaking with generator choices.

If Field.choices is provided as an iterator, consume it in __init__ instead
of using itertools.tee (which ends up holding everything in memory
anyway). Fixes a bug where deconstruct() was consuming the iterator but
bypassing the call to `tee`.

django/db/models/fields/__init__.py

@@ -9,7 +9,6 @@ import math
 import uuid
 import warnings
 from base64 import b64decode, b64encode
-from itertools import tee
 
 from django.apps import apps
 from django.db import connection
@@ -155,7 +154,9 @@ class Field(RegisterLookupMixin):
         self.unique_for_date = unique_for_date
         self.unique_for_month = unique_for_month
         self.unique_for_year = unique_for_year
-        self._choices = choices or []
+        if isinstance(choices, collections.Iterator):
+            choices = list(choices)
+        self.choices = choices or []
         self.help_text = help_text
         self.db_index = db_index
         self.db_column = db_column
@@ -405,7 +406,6 @@ class Field(RegisterLookupMixin):
         }
         attr_overrides = {
             "unique": "_unique",
-            "choices": "_choices",
             "error_messages": "_error_messages",
             "validators": "_validators",
             "verbose_name": "_verbose_name",
@@ -553,7 +553,7 @@ class Field(RegisterLookupMixin):
             # Skip validation for non-editable fields.
             return
 
-        if self._choices and value not in self.empty_values:
+        if self.choices and value not in self.empty_values:
             for option_key, option_value in self.choices:
                 if isinstance(option_value, (list, tuple)):
                     # This is an optgroup, so look inside the group for
@@ -848,14 +848,6 @@ class Field(RegisterLookupMixin):
         """
         return smart_text(self._get_val_from_obj(obj))
 
-    def _get_choices(self):
-        if isinstance(self._choices, collections.Iterator):
-            choices, self._choices = tee(self._choices)
-            return choices
-        else:
-            return self._choices
-    choices = property(_get_choices)
-
     def _get_flatchoices(self):
         """Flattened version of choices tuple."""
         flat = []

tests/migrations/test_state.py

@@ -595,6 +595,26 @@ class StateTests(TestCase):
         self.assertIsNot(old_model.food_mgr.model, new_model.food_mgr.model)
         self.assertIsNot(old_model.food_qs.model, new_model.food_qs.model)
 
+    def test_choices_iterator(self):
+        """
+        #24483 - ProjectState.from_apps should not destructively consume
+        Field.choices iterators.
+        """
+        new_apps = Apps(["migrations"])
+        choices = [('a', 'A'), ('b', 'B')]
+
+        class Author(models.Model):
+            name = models.CharField(max_length=255)
+            choice = models.CharField(max_length=255, choices=iter(choices))
+
+            class Meta:
+                app_label = "migrations"
+                apps = new_apps
+
+        ProjectState.from_apps(new_apps)
+        choices_field = Author._meta.get_field('choice')
+        self.assertEqual(list(choices_field.choices), choices)
+
 
 class ModelStateTests(TestCase):
     def test_custom_model_base(self):

tests/model_fields/tests.py

@@ -445,13 +445,6 @@ class ChoicesTests(test.TestCase):
         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):