I had another thought about [per-instance choices for `forms.ChoiceField`][oldpost].
Instead of overriding the `__init__` method of your form class, you could use
[an object with an `__iter__` method][iter] that returns a fresh iterable each time
it is called.

from django import forms

class LetterChoices(object):
“””Return a random list of max_choices letters of the alphabet.”””
def __init__(self, max_choices=3):
self.max_choices = max_choices

def __iter__(self):
import string, random

return iter((l, l) for l in random.sample(string.ascii_uppercase, self.max_choices))

class LetterForm(forms.Form):
“””Pick a letter from a small, random set.”””
letter = forms.ChoiceField(choices=LetterChoices())

I don’t know if I prefer that style to having a simple function – having to
instantiate the class seems wrong to me, I’d much rather use any callable as
the `choices` argument.

[oldpost]: http://reliablybroken.com/b/2009/03/per-instance-choices-for-djangos-formschoicefield/
[iter]: http://python.org/doc/current/library/stdtypes.html#typeiter