Fix HTMLFormRenderer with empty datetime values

rest_framework/renderers.py

@@ -345,7 +345,7 @@ def render_field(self, field, parent_style):
             except AttributeError:
                 format_ = api_settings.DATETIME_FORMAT
 
-            if format_ is not None:
+            if format_ is not None and field.value not in (None, ''):
                 # field.value is expected to be a string
                 # https://www.django-rest-framework.org/api-guide/fields/#datetimefield
                 field_value = field.value
@@ -358,10 +358,11 @@ def render_field(self, field, parent_style):
                     else datetime.datetime.strptime(field_value, format_)
                 )
 
-            # The format of an input type="datetime-local" is "yyyy-MM-ddThh:mm"
-            # followed by optional ":ss" or ":ss.SSS", so keep only the first three
-            # digits of milliseconds to avoid browser console error.
-            field.value = field.value.replace(tzinfo=None).isoformat(timespec="milliseconds")
+            if isinstance(field.value, datetime.datetime):
+                # The format of an input type="datetime-local" is "yyyy-MM-ddThh:mm"
+                # followed by optional ":ss" or ":ss.SSS", so keep only the first three
+                # digits of milliseconds to avoid browser console error.
+                field.value = field.value.replace(tzinfo=None).isoformat(timespec="milliseconds")
 
         if 'template' in style:
             template_name = style['template']

tests/test_renderers.py

@@ -566,6 +566,22 @@ def test_datetime_field_rendering_timezone_aware_datetime(self):
             "2024-12-23T09:55:30.345"  # Rendered in -06:00
         )
 
+    def test_datetime_field_rendering_empty_string_raises_no_error(self):
+        """
+        Regression test for #9927 (issue):
+        Ensures that an empty string value doesn't cause a ValueError
+        when the HTMLFormRenderer tries to parse it via fromisoformat.
+        """
+        self._assert_datetime_rendering("", "")
+
+    def test_datetime_field_rendering_none_value_raises_no_error(self):
+        """
+        Additional regression coverage for #9927:
+        Ensures that a None value, which is converted to an empty string
+        by as_form_field(), doesn't cause a ValueError when rendered.
+        """
+        self._assert_datetime_rendering(None, "")
+
 
 class TestHTMLFormRenderer(TestCase):
     def setUp(self):