#26166: Accessing a widget via an index on a BoundField results in unnecessary
iteration
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Reporter: seddonym | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Forms | Version: 1.9
Severity: Normal | Keywords: forms, boundfield
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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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.
--
Ticket URL: <https://code.djangoproject.com/ticket/26166>
Django <https://code.djangoproject.com/>
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