Per-instance choices for Django's forms.ChoiceField

I keep forgetting the details of how one customizes the choices in a forms.ChoiceField per instance. The forms.ChoiceField documentation says the required argument has to be an iterable but then moves straight to the next section.

Fortunately this was covered long ago by James Bennett when Django’s newforms module was introduced. See Getting dynamic model choices in newforms on Django Snippets.

The following form example has a field for picking a letter of the alphabet (works for Django 1.0). The choices are limited to 3 letters only, picked at random and different for each form instance:

from django import forms


def letter_choices(max_choices=3):
    """Return a random list of max_choices letters of the alphabet."""
    import string, random

    for l in random.sample(string.ascii_uppercase, max_choices):
        yield (l, l)


class LetterForm(forms.Form):
    """Pick a letter from a small, random set."""
    letter = forms.ChoiceField(choices=letter_choices())

    def __init__(self, *args, **kwargs):
        super(LetterForm, self).__init__(*args, **kwargs)
        self.fields['letter'].choices = letter_choices()

So that works. The LetterForm class uses the helper function letter_choices to provide the random choices, which actually returns a generator object rather than a list or tuple of choice pairs. I am relying on Django’s base ChoiceField class calling list() on the choices when each form is instantiated, so having letter_choices return a generator is hunky dory.

>>> f1 = LetterForm()
>>> f1['letter']
>>> print f1['letter']
<select name="letter" id="id_letter">
<option value="R">R</option>
<option value="N">N</option>
<option value="U">U</option>
</select>
>>> f2 = LetterForm()
>>> print f2['letter']
<select name="letter" id="id_letter">
<option value="O">O</option>
<option value="N">N</option>
<option value="T">T</option>
</select>
>>>

Now the only thing is… this example is not practical. Using genuinely random choices means that the valid choices on the form submitted by the user will be different to the valid choices on the form used to validate the user input on the next request, and this will likely raise a ValidationError.

Django feature suggestion: allow choices to be any iterable or callable, calling it as appropriate when instantiating the field. If it is callable you could pass a function which returns an iterable at that point, which would save one having to write an __init__ method for the form sub-class.