Fixed #36801 -- Avoided unnecessary calculation in construct_change_message().

`changed_field_labels` is only needed if there are changes to log, so move its
calculation, including the somewhat costly `translation_override()`, inside the
conditional that checks for changes. Also avoid reading `form.changed_data`
when it’s already bound to `changed_data`.

co-authored-by: Rodolfo Becerra <44782644+rodolvbg@users.noreply.github.com>

Author: 2 people

django/contrib/admin/utils.py

@@ -552,21 +552,21 @@ def construct_change_message(form, formsets, add):
     Translations are deactivated so that strings are stored untranslated.
     Translation happens later on LogEntry access.
     """
+    change_message = []
+    if add:
+        change_message.append({"added": {}})
     # Evaluating `form.changed_data` prior to disabling translations is
     # required to avoid fields affected by localization from being included
     # incorrectly, e.g. where date formats differ such as MM/DD/YYYY vs
     # DD/MM/YYYY.
-    changed_data = form.changed_data
-    with translation_override(None):
-        # Deactivate translations while fetching verbose_name for form
-        # field labels and using `field_name`, if verbose_name is not provided.
-        # Translations will happen later on LogEntry access.
-        changed_field_labels = _get_changed_field_labels_from_form(form, changed_data)
-
-    change_message = []
-    if add:
-        change_message.append({"added": {}})
-    elif form.changed_data:
+    elif changed_data := form.changed_data:
+        with translation_override(None):
+            # Deactivate translations while fetching verbose_name for form
+            # field labels and using `field_name`, if verbose_name is not
+            # provided. Translations will happen later on LogEntry access.
+            changed_field_labels = _get_changed_field_labels_from_form(
+                form, changed_data
+            )
         change_message.append({"changed": {"fields": changed_field_labels}})
     if formsets:
         with translation_override(None):