Django dependent ChoiceField options

On a recent project, I was looking to control the options available in a Django ChoiceField based on another form parameter (passed along with the response object).

The tricky part was that ChoiceFields by default perform their clean method using the choices key set on the field in forms.py. Because my choices were going to be very different from each other depening on the field passed to the form, I didn’t want to pass all possible choices to the ChoiceField and then use JavaScript to filter/remove them onload.

I did some searching on the interwebs and came across this blog post that talked about passing an init function to your form to essentially add your dependent field on the fly. It also talked about using a function as the choices keyword, which allows the list of choices to be determined at each load of the form.

The end result is a really beautiful solution for a problem I’m guessing a number of folks will come across. If you want to review a quick snippet of exactly how I setup my configuration, have a read through the code below. I’ve sanitized and simplified, but the overall example is highlighted well.

# somefile.json
{
    "key_one": ["", "A", "B", "C", "D"],
    "key_two": ["", "1", "2", "3"],
    "key_three": ["", "a1", "b2", "c3"]
}


# forms.py
try:
    import simplejson as json
except ImportError:
    import json

from django import forms


def getFieldChoices(key_field=None):
    """
    Return a tuple of field choices from a json file.

    """
    with open('somefile.json') as f:
        json_data = json.load(f)
        choices_list = []
        if key_field and key_field in json_data:
            fields = json_data[key_field]
            for field in fields:
                choices_list.append((field, field))
    return tuple(choices_list)


class MyForm(forms.Form):
    def __init__(self, *args, **kwargs):
        self._key_field = kwargs.pop('key_field', None)
        super(MyForm, self).__init__(*args, **kwargs)
        self.fields['my_choice_field'] = forms.ChoiceField(
            choices=getFieldChoices(self._key_field),
            required=False,
            initial='',
            widget=forms.Select()
        )

    # other standard fields go below...