Question about garbage collection

[Chris Angelico]

On Wed, 17 Jan 2024 at 01:45, Frank Millman via Python-list
<python-list at python.org> wrote:
>
> On 2024-01-16 2:15 PM, Chris Angelico via Python-list wrote:
> >
> > Where do you tend to "leave a reference dangling somewhere"? How is
> > this occurring? Is it a result of an incomplete transaction (like an
> > HTTP request that never finishes), or a regular part of the operation
> > of the server?
> >
>
> I have a class that represents a database table, and another class that
> represents a database column. There is a one-to-many relationship and
> they maintain references to each other.
>
> In another part of the app, there is a class that represents a form, and
> another class that represents the gui elements on the form. Again there
> is a one-to-many relationship.

I don't know when you'd be "done" with the table, so I won't try to
give an example, but I'll try this one and maybe it'll give some ideas
that could apply to both.

When you open the form, you initialize it, display it, etc, etc. This
presumably includes something broadly like this:

class Form:
    def __init__(self):
        self.elements = []

class Element:
    def __init__(self, form):
        self.form = form
        form.elements.append(self)

frm = Form(...)
Element(frm, ...) # as many as needed
frm.show() # present it to the user

This is a pretty classic refloop. I don't know exactly what your setup
is, but most likely it's going to look something like this. Feel free
to correct me if it doesn't.

The solution here would be to trap the "form is no longer being
displayed" moment. That'll be some sort of GUI event like a "close" or
"delete" signal. When that comes through (and maybe after doing other
processing), you no longer need the form, and can dispose of it. The
simplest solution here is: Empty out frm.elements. That immediately
leaves the form itself as a leaf (no references to anything relevant),
and the elements still refer back to it, but once nothing ELSE refers
to the form, everything can be disposed of.

> A gui element that represents a piece of data has to maintain a link to
> its database column object. There can be a many-to-one relationship, as
> there could be more than one gui element referring to the same column.

Okay, so the Element also refers to the corresponding Column. If the
Form and Element aren't in a refloop, this shouldn't be a problem.
However, if this is the same Table and Column that you referred to
above, that might be the answer to my question. Are you "done" with
the Table at the same time that the form is no longer visible? If so,
you would probably have something similar where the Form refers to the
Table, and the Table and Columns refer to each other... so the same
solution hopefully should work: wipe out the Table's list of columns.

> There are added complications which I won't go into here. The bottom
> line is that on some occasions a form which has been closed does not get
> gc'd.
>
> I have been trying to reproduce the problem in my toy app, but I cannot
> get it to fail. There is a clue there! I think I have just
> over-complicated things.

Definitely possible.

> I will start with a fresh approach tomorrow. If you don't hear from me
> again, you will know that I have solved it!
>
> Thanks for the input, it definitely helped.

Cool cool, happy to help.

ChrisA