Miscellany

class Optional(object):
    """
    Allows empty input and stops the validation chain from continuing.

    If input is empty, also removes prior errors (such as processing errors)
    from the field.
    """
    field_flags = ('optional', )

    def __call__(self, form, field):
        if not field.raw_data or isinstance(field.raw_data[0], basestring) and not field.raw_data[0].strip():
            field.errors[:] = []
            raise StopValidation()