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

Backport of 171f91d9ef5177850c2f12b26dd732785f6ac034 from main

1 files changed, 16 insertions, 1 deletions

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."""