`parent` is `None` until the element is retrieved via `__get_item__`

[me-kell]

When constructing panflute elements (e.g. Para) the content elements have the parent not set (i.e. it is set to None.

The parent is first set when the elements are requested through ListContainer.__getitem__

>>> from panflute import Para, Str
>>> s = Str("x")
>>> p = Para(s)
>>> print(s.parent)
None # the parent is None
>>> p.content[0] # accessing the content elements will set the parent
Str(x)
>>> print(s.parent)
Para(Str(x))

IMHO the state of the element should not depend on whether the element has been requested or not.

Is there any reason why the parent is not set when ListContainer.__set_item__ or ListContainer.insert are called?

This could be done calling ListContainer.attach at the end of ListContainer.__set_item__ or ListContainer.insert:

    def __setitem__(self, i, v):
        if isinstance(i, slice):
            v = (check_type(x, self.oktypes) for x in v)
        else:
            v = check_type(v, self.oktypes)
        self.list[i] = v
        attach(self.list[i], self.parent, self.location, i) # call attach to set the parent

    def insert(self, i, v):
        v = check_type(v, self.oktypes)
        self.list.insert(i, v)
        attach(self.list[i], self.parent, self.location, i) # call attach to set the parent

The same could be implemented for DictContainer.__setitem__

    def __setitem__(self, k, v):
        v = check_type(v, self.oktypes)
        self.dict[k] = v
        attach(self.dict[k], self.parent, self.location)