class ExpenseForm(forms.Form):
  CHOICES = (
    (11, 'Credit Card'),
    (12, 'Student Loans'),
    (13, 'Taxes'),
    (21, 'Books'),
    (22, 'Games'),
    (31, 'Groceries'),
    (32, 'Restaurants'),
  )
  date = forms.DateField()
  category = forms.ChoiceField(choices=CHOICES)

class ExpenseForm(forms.Form):
  CHOICES = (
    ('Debt', (
      (11, 'Credit Card'),
      (12, 'Student Loans'),
      (13, 'Taxes'),
    )),
    ('Entertainment', (
      (21, 'Books'),
      (22, 'Games'),
    )),
    ('Everyday', (
      (31, 'Groceries'),
      (32, 'Restaurants'),
    )),
  )
  date = forms.DateField()
  category = forms.ChoiceField(choices=CHOICES)

from django.db import models

class Category(models.Model):
  name = models.CharField(max_length=30)
  parent = models.ForeignKey('Category', on_delete=models.CASCADE, null=True)

  def __str__(self):
    return self.name

class Expense(models.Model):
  amount = models.DecimalField(max_digits=10, decimal_places=2)
  date = models.DateField()
  category = models.ForeignKey(Category, on_delete=models.CASCADE)

  def __str__(self):
    return self.amount

from functools import partial
from itertools import groupby
from operator import attrgetter

from django.forms.models import ModelChoiceIterator, ModelChoiceField

class GroupedModelChoiceIterator(ModelChoiceIterator):
  def __init__(self, field, groupby):
    self.groupby = groupby
    super().__init__(field)

  def __iter__(self):
    if self.field.empty_label is not None:
      yield ("", self.field.empty_label)
    queryset = self.queryset
    # Can't use iterator() when queryset uses prefetch_related()
    if not queryset._prefetch_related_lookups:
      queryset = queryset.iterator()
    for group, objs in groupby(queryset, self.groupby):
      yield (group, [self.choice(obj) for obj in objs])

class GroupedModelChoiceField(ModelChoiceField):
  def __init__(self, *args, choices_groupby, **kwargs):
    if isinstance(choices_groupby, str):
      choices_groupby = attrgetter(choices_groupby)
    elif not callable(choices_groupby):
      raise TypeError('choices_groupby must either be a str or a callable accepting a single argument')
    self.iterator = partial(GroupedModelChoiceIterator, groupby=choices_groupby)
    super().__init__(*args, **kwargs)

from django import forms
from .fields import GroupedModelChoiceField
from .models import Category, Expense

class ExpenseForm(forms.ModelForm):
  category = GroupedModelChoiceField(
    queryset=Category.objects.exclude(parent=None), 
    choices_groupby='parent'
  )

  class Meta:
    model = Expense
    fields = ('amount', 'date', 'category')