id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
26166	Accessing a widget via an index on a BoundField results in unnecessary iteration	David Seddon	nobody	"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."	Cleanup/optimization	closed	Forms	1.9	Normal	fixed	forms, boundfield		Accepted	0	0	0	0	0	0