def CustomField(parent_class, model_class, model_field):
    class _CustomField(parent_class):
        def __init__(self, *args, **kwargs):
            super(_CustomField, self).__init__(*args, **kwargs)
            field = model_class._meta.get_field(model_field).formfield()
            for key in field.__dict__:
                setattr(self, key, field.__dict__[key])
    return _CustomField

class UsernameField(CustomField(forms.CharField, User, 'username')):
    def __init__(self, *args, **kwargs):
        super(UsernameField, self).__init__(*args, **kwargs)
        self.label = 'Логин на сайте'
        self.help_text = 'Вы можете использовать символы: a-z, 0-9 и дефис.'