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
29893	Unexpected behavior of DetailView on a model defining __len__() returning False	Max	Max	"== Description 

When using Django's DetailView on a model defining a {{{__len__()}}} method, the model object is not always added to context. This is the case, when {{{__len__()}}} of the model object returns zero. The behavior is caused by {{{BaseDetailView.get()}}} calling {{{SingleObjectMixin.get_context_data()}}} which checks like this, if {{{self.object}}} was set before by{{{SingleObjectMixin.get_object()}}}: 
{{{
if self.object:
     # adds self.object to context
}}}

Because this check is happening implicitly and not against {{{None}}}, pythons default implementation calling {{{self.object.__len__()}}} ([https://docs.python.org/3/library/stdtypes.html#truth-value-testing]) causes this condition to evaluate to False, at least if {{{__len__()}}} returns zero. This leads to not having the wanted object in context data.


== Steps to reproduce:
Create an empty model defining a {{{__len__}}} method returning zero and a DetailView like this: 

{{{
class TestModel(models.Model):
    def __len__(self):
        return 0


class TestModelDetailView(DetailView):
    model = TestModel
    template_name = 'test.html'  # some template
}}}
After creating a TestModel object, the described behavior can be reproduced when calling the Detailview (url pattern like this {{{/test/<int:pk>/}}}).

== Proposed solution
Simply checking if self.object is the default value None prevents this not easy to find bug. 
{{{SingleObjectMixin.get_queryset()}}} also has a similar check 
{{{
if self.model:
    # some action
}}}
which does not seem to cause problems, because {{{self.model}}} is a class and no object. It should be considered to also check against the default value for self.model here, which is None.

"	Bug	closed	Generic views	2.1	Normal	wontfix		Max	Unreviewed	1	0	0	0	0	0