How does Django Know the Order to Render Form Fields?

If I have a Django form such as:

class ContactForm(forms.Form):
    subject = forms.CharField(max_length=100)
    message = forms.CharField()
    sender = forms.EmailField()

And I call the as_table() method of an instance of this form, Django will render the fields as the same order as specified above.

My question is how does Django know the order that class variables where defined?

(Also how do I override this order, for example when I want to add a field from the classe's init method?)

New to Django 1.9 is Form.field_order and Form.order_fields().

# forms.Form example
class SignupForm(forms.Form):

    password = ...
    email = ...
    username = ...

    field_order = ['username', 'email', 'password']


# forms.ModelForm example
class UserAccount(forms.ModelForm):

    custom_field = models.CharField(max_length=254)

    def Meta:
        model = User
        fields = ('username', 'email')

    field_order = ['username', 'custom_field', 'password']

[NOTE: this answer is now pretty completely outdated - please see the discussion below it, and more recent answers].

If f is a form, its fields are f.fields, which is a django.utils.datastructures.SortedDict (it presents the items in the order they are added). After form construction f.fields has a keyOrder attribute, which is a list containing the field names in the order they should be presented. You can set this to the correct ordering (though you need to exercise care to ensure you don't omit items or add extras).

Here's an example I just created in my current project:

class PrivEdit(ModelForm):
    def __init__(self, *args, **kw):
        super(ModelForm, self).__init__(*args, **kw)
        self.fields.keyOrder = [
            'super_user',
            'all_districts',
            'multi_district',
            'all_schools',
            'manage_users',
            'direct_login',
            'student_detail',
            'license']
    class Meta:
        model = Privilege