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
7221	TypeErrors from Field.get_db_prep_lookup() get suppressed	Marty Alchin	nobody	"The custom model fields documentation for [http://www.djangoproject.com/documentation/custom_model_fields/#get-db-prep-lookup-self-lookup-type-value get_db_prep_lookup()] show the ability to raise a `TypeError` as a way to prevent certain types of lookups from taking place. In practice, however, it seems this error gets suppressed somewhere along the line, result in the QuerySet returning an empty list.

The query doesn't actually get executed in the database, as it's possible to observe this behavior even without having a true database table backing up the model. Also, while `TypeError` gets eaten, `ValueError` passes through just fine. I haven't tried all error types, but `AttributeError` also gets suppressed in the same manner as `TypeError`.

Also of interest, when manually iterating over a QuerySet that should raise a `TypeError`, it does in fact raise the error appropriately. However, when simply calling `list()` on it, the bug shows up. The following interpreter session illustrates the problem and its many peculiarities.

{{{
#!python
>>> from django.db import models

>>> class TypeErrorField(models.TextField):
...     def get_db_prep_lookup(self, lookup_type, value):
...         raise TypeError(""Lookups are prohibited."")

>>> class ValueErrorField(models.TextField):
...     def get_db_prep_lookup(self, lookup_type, value):
...         raise ValueError(""Lookups are prohibited."")

>>> class Test(models.Model):
...     type_error = TypeErrorField()
...     value_error = ValueErrorField()
...     class Meta:
...         app_label = 'test'

>>> Test.objects.filter(type_error='test')
[]
>>> list(Test.objects.filter(type_error='test'))
[]
>>> [x for x in Test.objects.filter(type_error='test')]
Traceback (most recent call last):
  ...
TypeError: Lookups are prohibited.

>>> Test.objects.filter(value_error='test')
Traceback (most recent call last):
  ...
ValueError: Lookups are prohibited.
}}}

Whether the documentation is wrong, or the documentation, I'm not sure, but there's certainly a disconnect here. If it matters, this was tested on Windows Vista, Python 2.5, SQLite. I've tried to trace through the execution, but I was unable to find anything that would be suppressing the error."		closed	Database layer (models, ORM)	dev		fixed			Accepted	0	0	1	0	0	0