How to use the hl7apy.exceptions.ChildNotValid function in hl7apy

f = m.rsp_k21_query_response.pid.add_field('PID_3')  # it is a complex datatype field
        with self.assertRaises(ChildNotValid):
            f.value = ST('aaa')

        m = Message('RSP_K21', reference=self.rsp_k21_mp, validation_level=VALIDATION_LEVEL.STRICT)
        f = m.rsp_k21_query_response.pid.add_field('PID_1')
        f.value = SI(1)
        self.assertEqual(f.to_er7(), '1')
        f.value = SI(2)
        self.assertEqual(f.to_er7(), '2')
        with self.assertRaises(ChildNotValid):
            f.value = ST('aaa')

        m = Message('RSP_K21', reference=self.rsp_k21_mp, validation_level=VALIDATION_LEVEL.STRICT)
        f = m.rsp_k21_query_response.pid.add_field('PID_3')  # it is a complex datatype field
        with self.assertRaises(ChildNotValid):
            f.value = ST('aaa')

if isinstance(value, ElementProxy):
            value = value[0].to_er7()

        name = name.upper()
        reference = None if name is None else self.element.find_child_reference(name)
        child_ref, child_name = (None, None) if reference is None else (reference['ref'], reference['name'])

        if isinstance(value, basestring):  # if the value is a basestring, parse it
            child = self.element.parse_child(value, child_name=child_name, reference=child_ref)
        elif isinstance(value, Element):  # it is already an instance of Element
            child = value
        elif isinstance(value, BaseDataType):
            child = self.create_element(name, False, reference)
            child.value = value
        else:
            raise ChildNotValid(value, child_name)

        if child.name != child_name:  # e.g. message.pid = Segment('SPM') is forbidden
            raise ChildNotValid(value, child_name)

        child_to_remove = self.child_at_index(child_name, index)

        if child_to_remove is None:
            self.append(child)
        else:
            self.replace_child(child_to_remove, child)

        # a set has been called, change the temporary parent to be the actual one
        self.element.set_parent_to_traversal()

def find_child_reference(self, name):
        name = name.upper()
        if isinstance(self.structure_by_name, MutableMapping):
            element = self.structure_by_name.get(name) or self.structure_by_longname.get(name)
        else:
            element = None

        if element is None:  # not found in self.structure
            element = find_reference(name, self.child_classes.values(), self.version)
            if element is None:
                raise ChildNotFound(name)
            # it means that the child exists but it's not valid for the Element (e.g. Field('pid_3').ce_1)
            if self.structure_by_name is not None:
                raise ChildNotValid(name, self)
        return element

"""
        name = name.upper()
        element = self.structure_by_name.get(name, None) or self.structure_by_longname.get(name, None)

        if element is None:  # not found in self.structure
            if self.allow_infinite_children and _valid_child_name(name, self.name):
                if _valid_z_field_name(name):
                    datatype = 'ST'
                else:
                    datatype = 'varies'

                element = {'cls': Field, 'name': name, 'ref': ('leaf', None, datatype, None, None, -1)}
            else:
                element = find_reference(name, self.child_classes.values(), self.version)
                if element:
                    raise ChildNotValid(name, self)
                else:
                    raise ChildNotFound(name)
        return element

if self.element._is_valid_child(child):
            if child.parent != self.element and child.traversal_parent != self.element:  # avoid infinite recursion
                child.parent = self.element
            else:
                # if validation is strict, check the child cardinality
                if Validator.is_strict(self.element.validation_level):
                    min_rep, max_rep = self.element.repetitions.get(child.name, (0, -1))
                    if len(self.indexes.get(child.name, [])) + 1 > int(max_rep) and max_rep > -1:
                        raise MaxChildLimitReached(self.element, child, max_rep)
                if self.element.validation_level != child.validation_level:
                    raise OperationNotAllowed('Cannot add a child with a different validation_level')
                if self.element.version != child.version:
                    raise OperationNotAllowed('Cannot add a child with a different HL7 version')
                return True
        else:
            raise ChildNotValid(child, self.element)
        return False

def find_child_reference(self, name):
        name = name.upper()
        if isinstance(self.structure_by_name, MutableMapping):
            element = self.structure_by_name.get(name) or self.structure_by_longname.get(name)
        else:
            element = None
        if element is None:  # not found in self.structure
            if _valid_z_segment_name(name):
                element = {'cls': Segment, 'name': name, 'ref': ('sequence', ())}
            else:
                element = find_reference(name, self.child_classes.values(), self.version)
                if Validator.is_strict(self.validation_level):  # cannot be created if validation is strict
                    raise ChildNotValid(name, self)
        return element

name = name.upper()
        reference = None if name is None else self.element.find_child_reference(name)
        child_ref, child_name = (None, None) if reference is None else (reference['ref'], reference['name'])

        if isinstance(value, basestring):  # if the value is a basestring, parse it
            child = self.element.parse_child(value, child_name=child_name, reference=child_ref)
        elif isinstance(value, Element):  # it is already an instance of Element
            child = value
        elif isinstance(value, BaseDataType):
            child = self.create_element(name, False, reference)
            child.value = value
        else:
            raise ChildNotValid(value, child_name)

        if child.name != child_name:  # e.g. message.pid = Segment('SPM') is forbidden
            raise ChildNotValid(value, child_name)

        child_to_remove = self.child_at_index(child_name, index)

        if child_to_remove is None:
            self.append(child)
        else:
            self.replace_child(child_to_remove, child)

        # a set has been called, change the temporary parent to be the actual one
        self.element.set_parent_to_traversal()

>>> c.add(s)
        >>> c.add(s2)
        >>> print(c.to_er7())
        EXAMPLE_ID&&&ALT_ID
        """
        # base datatype components can't have more than one child
        if self.name and is_base_datatype(self.datatype, self.version) and \
                len(self.children) >= 1:
            raise MaxChildLimitReached(self, obj, 1)

        # the name is different from the datatype (i.e. the name has been forced to be equal to the datatype)
        try:
            if obj.name and obj.name != obj.datatype:
                try:
                    if not _valid_child_name(obj.name, self.datatype):
                        raise ChildNotValid(obj.name, self)
                except AttributeError:
                    pass
        except ChildNotFound:  # obj.datatype causes ChildNotFound for some Elements (Message, Groups etc)
            raise ChildNotValid(obj, self)

        return super(Component, self).add(obj)