Fixed #34899 -- Added blank choice to forms' callable choices lazily.

tests/model_forms/tests.py

@@ -23,6 +23,7 @@ from django.forms.models import (
 from django.template import Context, Template
 from django.test import SimpleTestCase, TestCase, ignore_warnings, skipUnlessDBFeature
 from django.test.utils import isolate_apps
+from django.utils.choices import BlankChoiceIterator
 from django.utils.deprecation import RemovedInDjango60Warning
 
 from .models import (
@@ -2012,6 +2013,38 @@ class ModelFormBasicTests(TestCase):
             ),
         )
 
+    @isolate_apps("model_forms")
+    def test_callable_choices_are_lazy(self):
+        call_count = 0
+
+        def get_animal_choices():
+            nonlocal call_count
+            call_count += 1
+            return [("LION", "Lion"), ("ZEBRA", "Zebra")]
+
+        class ZooKeeper(models.Model):
+            animal = models.CharField(
+                blank=True,
+                choices=get_animal_choices,
+                max_length=5,
+            )
+
+        class ZooKeeperForm(forms.ModelForm):
+            class Meta:
+                model = ZooKeeper
+                fields = ["animal"]
+
+        self.assertEqual(call_count, 0)
+        form = ZooKeeperForm()
+        self.assertEqual(call_count, 0)
+        self.assertIsInstance(form.fields["animal"].choices, BlankChoiceIterator)
+        self.assertEqual(call_count, 0)
+        self.assertEqual(
+            form.fields["animal"].choices,
+            models.BLANK_CHOICE_DASH + [("LION", "Lion"), ("ZEBRA", "Zebra")],
+        )
+        self.assertEqual(call_count, 1)
+
     def test_recleaning_model_form_instance(self):
         """
         Re-cleaning an instance that was added via a ModelForm shouldn't raise