How to proceed to add a custom validator to a newforms field : you just need to create a new class derivated from forms.YourField with a custom clean method. Do not forget the line super(UserField, self).clean(value) ; in our case, it verifies the field attributes : min_length, max_length or required. More explications (in French) : [des validateurs personnalisés pour Django](http://www.aozeo.com/blog/67-django-newforms-validateurs-personnalises)
You may want to access to "global" errors in your template, typically when you have a custom `clean()` method which checks several fields. `{{ form.errors }}` gives an < ul >, which is often not what you want, and `{{ form.errors|join:", " }}` iterates in the keys of `errors` (which are the names of fields, uninteresting). The global `errors`'s key is "__all__", and Django doesn't allow us to do `{{ form.errors.__all__ }}`. This method returns an array, like a classic `{{ form.field.errors }}`, and can be used with a join : `{{ form.get_global_errors|join:", " }}`.