[5.0.x] Fixed #34899 -- Added blank choice to forms' callable choices lazily.

Backport of 171f91d9ef5177850c2f12b26dd732785f6ac034 from main

2 files changed, 19 insertions, 8 deletions

diff --git a/django/db/models/fields/__init__.py b/django/db/models/fields/__init__.py
index 6174b7bc98..205a41c193 100644
--- a/django/db/models/fields/__init__.py
+++ b/django/db/models/fields/__init__.py
@@ -16,6 +16,7 @@ from django.db.models.constants import LOOKUP_SEP
 from django.db.models.query_utils import DeferredAttribute, RegisterLookupMixin
 from django.utils import timezone
 from django.utils.choices import (
+    BlankChoiceIterator,
     CallableChoiceIterator,
     flatten_choices,
     normalize_choices,
@@ -1055,14 +1056,9 @@ class Field(RegisterLookupMixin):
         as <select> choices for this field.
         """
         if self.choices is not None:
-            choices = list(self.choices)
             if include_blank:
-                blank_defined = any(
-                    choice in ("", None) for choice, _ in self.flatchoices
-                )
-                if not blank_defined:
-                    choices = blank_choice + choices
-            return choices
+                return BlankChoiceIterator(self.choices, blank_choice)
+            return self.choices
         rel_model = self.remote_field.model
         limit_choices_to = limit_choices_to or self.get_limit_choices_to()
         choice_func = operator.attrgetter(
diff --git a/django/utils/choices.py b/django/utils/choices.py
index 54dbdcb3aa..7f40bce510 100644
--- a/django/utils/choices.py
+++ b/django/utils/choices.py
@@ -1,10 +1,11 @@
 from collections.abc import Callable, Iterable, Iterator, Mapping
-from itertools import islice, zip_longest
+from itertools import islice, tee, zip_longest
 
 from django.utils.functional import Promise
 
 __all__ = [
     "BaseChoiceIterator",
+    "BlankChoiceIterator",
     "CallableChoiceIterator",
     "flatten_choices",
     "normalize_choices",
@@ -34,6 +35,20 @@ class BaseChoiceIterator:
         )
 
 
+class BlankChoiceIterator(BaseChoiceIterator):
+    """Iterator to lazily inject a blank choice."""
+
+    def __init__(self, choices, blank_choice):
+        self.choices = choices
+        self.blank_choice = blank_choice
+
+    def __iter__(self):
+        choices, other = tee(self.choices)
+        if not any(value in ("", None) for value, _ in flatten_choices(other)):
+            yield from self.blank_choice
+        yield from choices
+
+
 class CallableChoiceIterator(BaseChoiceIterator):
     """Iterator to lazily normalize choices generated by a callable."""