Hi Vincent!
Because it’s a broad question, I’ll provide a broad answer.
This is most likely the result of reloading without freeing resources
claimed by Qt. When you reload, Python does a good job garbage collecting
all of its items, but it cannot know the order in which Qt’s objects should
be freed. In this case, it’s likely the assigned widget to your proxy need
to be cleaned up first; but as Python doesn’t know of this relationship, it
cannot take that information into account during garbage collection, which
would in that case delete something that Qt then tried to delete again,
which in C++ land would be undefined. Undefined means it may crash, or it
may not. Which means the first 3 times you try, you’re in luck. And on the
4th you’re not. Undefined doesn’t necessarily mean random though, so it may
even consistently not-crash the first 3, and crash consistently on the 4th.
You can avoid situations like this altogether with a little programming
hygene. What I typically do is two things.
1. Anything you install needs an equivalent uninstall
2. Instead of freeing with reload(), free with sys.modules.pop
The uninstall in this case could consist of un-assigning the widgets to the
proxy before freeing. Qt is able to automatically free objects if they form
a hierarchy, like when you assign widgets to a layout. I would expect it to
also handle cases where you assign a widget like in your example, but odds
are it doesn’t in which case you need to do it yourself.
Generally, I structure my projects into an install() and uninstall()
function, where uninstall() does everything install() does in reverse;
including making connections to databases, or creating menus in Maya or
loading images and fonts off of disk etc. On calling uninstall() I expect a
Maya in the exact same condition that it was when I first called install().
This could happen either at a package, module and/or class level.
Freeing with sys.modules.pop("mymodule") is relevant when you work with
multiple Python modules that reference each other. You’ve probably already
found that if you reload a module that imports another module, that
imported module isn’t reloaded. It would hold onto any references it made -
including widgets that the parent module made use of - which means the
module you reload would re-import that persistent module and thus gain
access to whatever data it previously held. You could work around it by
reloading that module too, but the issue arises when modules form a
hierarchy and worse yet when there’s diamond or cyclic dependency between
modules.
In that case you’ve probably also found that the *order* in which you
reload becomes important and then down the rabbit hole you go.
To solve this you can instead divide the work of unloading and loading by
removing entries from sys.modules.
# For exampleimport mymodule
import sysfor module in list(sys.modules):
if module.startswith("mymodule"):
sys.modules.pop(module)
import mymodule
The .pop() has the same effect as reload, except it doesn’t automatically
re-import it, the .startswith ensures that any sub-modules are taken into
account as well. Then when you next import, it will import it in whatever
order it was imported the first time around. This also guarantees that the
modules you’ve now imported won’t contain any references from before and
that it’s an entirely clean slate.
It will also make apparent which of those references you were dependent on
and throw lots more errors due to things actually disappearing and not
lingering. These are the errors you want to get rid of.
Somewhat general advice, but having followed this method for many years I
haven’t had these issues when iterating on modules or packages in Maya (or
any embedded environment) and certainly no crashes because of it.
For more specific advice about this specific crash, maybe try and replicate
it in mayapy, in e.g. a loop that imports it over and over until it
crashes. Such that you can cause a crash fast and consistently. Once you’ve
got it, reduce it down to the few lines that cause it and take things from
there.
On Sun, 8 Mar 2020 at 18:26, vince touache <fruityfr...@gmail.com> wrote:
> hello,
>
> I'm wrapping a *QWidget *inside a *QGraphicsItem*, which is in turn used
> in a *QGraphicsView/Scene *(all using *PySide2*)
> To do so, I'm using a *QGraphicsProxyWidget *(first time I'm using it),
> like below :
> *proxy = QGraphicsProxyWidget(self) # I'm inside my QGraphicsItem, so
> self == QGraphicsItem*
> *proxy.setWidget(myWidget)*
>
> It works, but after 4 reloads/instanciating my view, it crashes my
> application (in that case, Maya).
> *import graphic_view;reload(graphic_view)*
> *v = graphic_view.graphic_view()*
> *v.close()*
>
> *# ---- x1 is ok ------*
>
> *# ---- x2 is ok ------*
>
> *# ---- x3 is ok ------*
>
> *# ---- x4 ==> crashing ------*
>
> I noticed that if I use another variable to store my new instance (x
> instead of v, they y, z, w, ....) it doesn't crash
>
> So I guess it has to do with how the memory is released (or not released
> where it should ! Maybe some parts are, some other not, which would mess
> with the garbage collection mechanism ?).
>
> If anyone has a better understanding of proxy widgets and what I am doing
> wrong, that'd be really appreciated. And more generally speaking, I always
> feel a bit powerless when I face situations like this : I usually uncomment
> pieces of code until I isolate the issue, when I get crashes, but that
> seems really like a cheap debugging workflow. But no way to attach debugger
> or anything like that, since the crash will simply close the application.
> How do you guys deal with this kind of problem ?
>
> Thank you !
>
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