def _qt_set_leaf_data(self, qvar):
    if VERBOSE_PREF:
        print('')
        print('+--- [pref.qt_set_leaf_data]')
        print('[pref.qt_set_leaf_data] qvar = %r' % qvar)
        print('[pref.qt_set_leaf_data] _intern.name=%r' % self._intern.name)
        print('[pref.qt_set_leaf_data] _intern.type_=%r' % self._intern.get_type())
        print('[pref.qt_set_leaf_data] type(_intern.value)=%r' % type(self._intern.value))
        print('[pref.qt_set_leaf_data] _intern.value=%r' % self._intern.value)
    if self._tree.parent is None:
        raise Exception('[Pref.qtleaf] Cannot set root preference')
    if self.qt_is_editable():
        new_val = '[Pref.qtleaf] BadThingsHappenedInPref'
        if self._intern.value == PrefNode:
            raise Exception('[Pref.qtleaf] Qt can only change leafs')
        elif self._intern.value is None:
            def cast_order(var, order=[bool, int, float, six.text_type]):
                for type_ in order:
                    try:
                        ret = type_(var)
                        return ret
                    except Exception:
                        continue
            new_val = cast_order(six.text_type(qvar))
        self._intern.get_type()
        if isinstance(self._intern.value, bool):
            print('qvar = %r' % (qvar,))
            new_val = util_type.smart_cast(qvar, bool)
            print('new_val = %r' % (new_val,))
        elif isinstance(self._intern.value, int):
            new_val = int(qvar)
        elif self._intern.get_type() in util_type.VALID_FLOAT_TYPES:
            new_val = float(qvar)
        elif isinstance(self._intern.value, six.string_types):
            new_val = six.text_type(qvar)
        elif isinstance(self._intern.value, PrefChoice):
            new_val = six.text_type(qvar)
            if new_val.upper() == 'NONE':
                new_val = None
        else:
            try:
                type_ = self._intern.get_type()
                if type_ is not None:
                    new_val = type_(six.text_type(qvar))
                else:
                    new_val = six.text_type(qvar)
            except Exception:
                raise NotImplementedError(
                    ('[Pref.qtleaf] Unknown internal type. '
                     'type(_intern.value) = %r, '
                     '_intern.get_type() = %r, ')
                    % type(self._intern.value), self._intern.get_type())
        if isinstance(new_val, six.string_types):
            if new_val.lower() == 'none':
                new_val = None
            elif new_val.lower() == 'true':
                new_val = True
            elif new_val.lower() == 'false':
                new_val = False
        if VERBOSE_PREF:
            print('---')
            print('[pref.qt_set_leaf_data] new_val=%r' % new_val)
            print('[pref.qt_set_leaf_data] type(new_val)=%r' % type(new_val))
            print('L____ [pref.qt_set_leaf_data]')
        return self._tree.parent.pref_update(self._intern.name, new_val)
    return 'PrefNotEditable'