Opened 9 years ago
Closed 8 years ago
#26166 closed Cleanup/optimization (fixed)
Accessing a widget via an index on a BoundField results in unnecessary iteration
Reported by: | David Seddon | Owned by: | nobody |
---|---|---|---|
Component: | Forms | Version: | 1.9 |
Severity: | Normal | Keywords: | forms, boundfield |
Cc: | Triage Stage: | Accepted | |
Has patch: | no | Needs documentation: | no |
Needs tests: | no | Patch needs improvement: | no |
Easy pickings: | no | UI/UX: | no |
Description
If you access a widget via its index on a BoundField
, the BoundField
will iterate over its entire set of widgets. This can cause large unnecessary overheads if you are accessing all the widgets in this way on, say, a ModelChoiceField
with a large queryset, as subwidgets for the entire queryset will be generated for each widget.
This is related to this ticket: https://code.djangoproject.com/ticket/22745, and the fix here: https://github.com/django/django/commit/5e06fa1469180909c51c07151692412269e51ea3
The specific example where this caused problems for me was in a custom filter that helped pass the associated instance along with its associated checkbox widget in the template. (http://stackoverflow.com/a/27545910/1005499)
Example:
instance_widgets = [] index = 0 for instance in bound_field.field.queryset.all(): # Accessing the widget here will generate subwidgets for items in the queryset widget = copy(bound_field[index]) instance_widgets.append((instance, widget)) index += 1
If we store the instantiated subwidgets on the BoundField instance then we reduce this overhead:
# django/forms/boundfield.py ... class BoundField(object): ... def __getitem__(self, idx): if not isinstance(idx, six.integer_types): raise TypeError # Prevent unnecessary reevaluation when accessing BoundField's attrs # from templates. if not hasattr(self, '_stored_subwidgets'): self._stored_subwidgets = list(self.__iter__()) return self._stored_subwidgets[idx]
I'm not sure if this is necessarily the correct thing to do, but I thought it worth reporting. An alternative would be just to give a warning if widgets are being accessed in this way.
Change History (4)
comment:1 by , 9 years ago
Triage Stage: | Unreviewed → Accepted |
---|
comment:2 by , 8 years ago
It might be worth it to effectively cache BoundField.__iter__()
but in your case, can't you use zip
as a workaround:
for widget, instance in zip(bound_field, bound_field.field.queryset.all()): instance_widgets.append((instance, copy(widget)))
I'm not sure about this either, but if we have a tested patch we can evaluate it.