`construct_change_message`call `form.changed_data` after check if is add

[Rodolfo Becerra]

This is the currently code of the method construct_change_message of django.contrib.admin.utils

def construct_change_message(form, formsets, add):
    """
    Construct a JSON structure describing changes from a changed object.
    Translations are deactivated so that strings are stored untranslated.
    Translation happens later on LogEntry access.
    """
    # 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:
        change_message.append({"changed": {"fields": changed_field_labels}})

If you notice that the form.changed_data property is being called unnecessarily before checking whether it is an add behavior. I propose call the changed_data = form.changed_data after check if is add.

If I am accepted, I would like to create the PR myself.

[Jacob Walls]

Hi Rodolfo. So changed_data is a cached property to avoid having to reason about this in various places. Given that, I'm inclined to close as needsinfo pending a benchmark showing this is worth devoting a review cycle to. This is generally how we handle all optimization proposals.

[Adam Johnson]

Rodolfo created this ticket at my encouragement, after he showed me a diff of the change he meant. I think the description didn't quite do it justice, so I've created a PR to show what he meant. I think it's a pretty clear optimization.

[Jacob Walls]

Thanks, that helps.

[Jacob Walls <jacobtylerwalls@…>]

In a77e541:

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@…>