ModelChoiceIterator doesn't adjust its length to account for an empty item.

[facundo.olano@…]

In forms/models.py, the ModelChoiceIterator defines an __iter__ method which yields an empty entry if empty_label is not None, but the __len__ method always returns the length of the queryset, no matter if the empty choice is present.

def __iter__(self):
    if self.field.empty_label is not None:
        yield (u"", self.field.empty_label)
    if self.field.cache_choices:
        if self.field.choice_cache is None:
            self.field.choice_cache = [
                self.choice(obj) for obj in self.queryset.all()
            ]
        for choice in self.field.choice_cache:
            yield choice
    else:
        for obj in self.queryset.all():
            yield self.choice(obj)

def __len__(self):
    return len(self.queryset)

This can lead to erratic behavior when iterating. For example, the following template code wont hold the expected output, because the forloop.last will be True before for the second to last item, causing the semicolon to render on the last line and not in the previous one.

{% for choice_id, choice_name in my_form.my_field.field.choices %}
    {{ choice_id }} - {{ choice_name }} {% if not forloop.last %};{% endif %}
{% endfor %}

This would produce an output like:

 - -----;
1 - One;
2 - Two
3 - Three;

[Stephen Burrows]

The issue is valid. Personally, I don't like using booleans as ints, but apparently it's part of the spec, not an implementation detail [1], so I can't complain super much. Pretty sure this needs tests, though.

[1] ​http://stackoverflow.com/questions/2764017/is-false-0-and-true-1-in-python-an-implementation-detail-or-is-it-guarante

[Klaas van Schelven]

I've added a test and bike-shedded the implementation; available as pull request on github

​https://github.com/django/django/pull/798

[Claude Paroz <claude@…>]

In 1c11ee63459c4362affbd6a20f2ddf9c2ecd8dce:

Fixed #18829 -- Fixed ModelChoiceIterator length

Thanks facundo.olano at gmail.com for the report and thikonom for
the initial patch.