Add empty value choice for ChoiceFilter

[rpkilby]

Adds empty and null value handling for ChoiceFilter, which should fix #45 and resolve #261.

• Adds FILTERS_EMPTY_CHOICE_LABEL, FILTERS_NULL_CHOICE_LABEL, and FILTERS_NULL_CHOICE_VALUE settings, which allow you to globally set the empty/null choice labels and the null choice value.
  • Empty choice value is hardcoded as there shouldn't need to be a reason to override it.
  • Null choice value defaults to 'null', which should be a JS-friendly magic value.
• Updated ChoiceFilter, which now takes corresponding empty_label, null_label, and null_value arguments. This is based on ModelChoiceField.empty_label.
  • empty_label and null_label may be set to None to disable those choices.
  • empty_label defaults to ---------, which maintains consistency w/ Django. This can be overridden globally or per filter.
  • null_label defaults to None, which means users must opt into using it.

TODO:

• Add docs
• Add tests
  • override empty_label value
  • test empty_label when None
  • test w/ other widgets (ChecboxSelectMultiple) There isn't anything to test here?
  • test w/ various filters (OrderingFilter, AllValuesFilter)

[carltongibson]

OK. Good.

This handles the ANY or empty case. If you could just give the example of adjusting choices and overriding filter to add a NONE — similar to the example in #261 — I'd call this closed I think.

def __init__(...):
    super(...)...

   choices = ('NONE', 'None') + self.choices

def filter(self, qs, value):
    if value == 'NONE':
        return qs.filter(type_id=None)
   else:
      return super(...).filter(qs, value)

...or such.

Make sense?

[rpkilby]

@carltongibson - I've updated the PR. I'll add docs/test if this all looks good.

[rpkilby]

Hi @carltongibson - the PR has been redone w/ docs and tests

[carltongibson]

Conflict in docs/ref/settings.txt

[rpkilby]

Rebased.

[carltongibson]

https://travis-ci.org/carltongibson/django-filter/jobs/172588858#L593-L595

  File "/home/travis/build/carltongibson/django-filter/tests/test_filters.py", line 372, in ChoiceFilterTests
    @override_settings(
NameError: name 'override_settings' is not defined

[rpkilby]

Just noticed that :(

In the middle of another rebase.

[rpkilby]

Already - tests should be good now.