enhancement request: require a or b but not both

[jvanasco]

There doesn't seem to be a way to require A or B, but not A&B.

This could be achieved with RequireEmptyIfPresent/RequireEmptyIfMissing, which would be the same logic as RequireIfMissing - but tests for an empty value.

[jvanasco]

After toying around, I found this approach better:

class OnlyOneOf(FormValidator):
    # Field that only one of is allowed
    only_one_ofs = None
    not_empty = None
    __unpackargs__ = ('only_one_ofs', )

    messages = {
        'empty': _("You must submit one and only one of these linked fields."),
        'invalid': _("You may submit only one of these linked fields."),
    }

    def _to_python(self, value_dict, state):
        is_empty = self.field_is_empty
        presence = [not is_empty(value_dict.get(field)) for field in self.only_one_ofs]
        total_present = presence.count(True)
        if not total_present and self.not_empty:
            raise Invalid(
                _('You must provide a value for one of the fields: %s') % ', '.join(["`%s`" % field for field in self.only_one_ofs]),
                value_dict,
                state,
                error_dict = dict([(field,
                                    Invalid(self.message('empty', state),
                                            value_dict.get(field),
                                            state
                                            )
                                    )
                                    for field in self.only_one_ofs
                                   ]
                                  )
            )
        if total_present > 1:
            raise Invalid(
                _('You may only provide a value for one of the fields: %s') % ', '.join(["`%s`" % field for field in self.only_one_ofs]),
                value_dict,
                state,
                error_dict = dict([(field,
                                    Invalid(self.message('invalid', state),
                                            value_dict.get(field),
                                            state
                                            )
                                    )
                                    for field in self.only_one_ofs
                                   ]
                                  )
            )
        return value_dict

• updated 2015/09/11 - supply True to the not_empty kwarg (defaults to None) will control when errors are raised. This allows for the fieldset to be optional or required.