class Post(models.Model):
    SETS = sets_choices()
...
    set = models.CharField(max_length=120, choices=SETS)

class MyForm(Form)
    def __init__(self, *args, **kwargs):
        choices = [bla-bla]
        self.fields['my_field'] = forms.ChoiceField(choices = choices, required=False)

from django import forms
from django.contrib import admin
from myapp.models import Person
class PersonForm(forms.ModelForm):
    class Meta:
        model = Person
        exclude = ['name']
class PersonAdmin(admin.ModelAdmin):
    form = PersonForm

class SetForm(forms.ModelForm):
    choices =  sets_choices()
    set =  forms.ChoiceField(choices = choices, required=False)
class FotoreportAdmin(admin.ModelAdmin):     
    list_display = ('title', 'date_creation', 'publish')
    form = SetForm

class SetForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        choices = sets_choices()
        self.fields['set'] =  forms.ChoiceField(choices = choices)