[PyQt] what is the difference between QMenu() and QMenu(self)

[Jian Ma]

Hi Mike,

I agree with your opion. Actually, I did backtrace. The message showed like
"~qobject, qobject.cpp can not find." I guess that the pyqt try to delete an
deleted object.

If you have other ideas, please let me know.

Thanks!

Jian

On Fri, Jul 29, 2011 at 7:44 PM, Mike Ramirez <gufymike at gmail.com> wrote:

> On Friday, July 29, 2011 04:20:27 PM Algis Kabaila wrote:
> > On Sat, 30 Jul 2011 08:18:58 AM Jian Ma wrote:
> > > Hi all,
> > >
> > > I created a QMenu object in a class. When I created it with menu =
> > > QMenu(self) and run the program, it sometimes got the "segmentation
> > > fault" after exit the program. When I created the object with menu =
> > > QMenu(), it didn't get this problem. I guess the reason is about the
> > > QMenu object ownership. In one case, the object is owned by PyQt. In
> > > other case, it is owned by Qt. I am not sure my guess. Can someone give
> > > some explanation? Below is some code segment.
> > >
> > > class DnDMenuListWidget(QListWidget):
> > >     ...
> > >     ...
> > >
> > >     def dropEvent(self, event):
> > >         ...
> > >         ...
> > >         menu = QMenu(self)    # using QMenu(self)
> > >         # menu = QMenu()      #  using QMenu()
> > >         ...
> > >         ...
> > >
> > > Thanks!
> >
> > In Python in a class instantiation the instance name is automatically
> > passed in a parameter, usually in a named variable "self", so if QMenu is
> > a method of a class, a varible named "self" needs to be specified, hence
> > QMenu(self).
> >
> > If Qmenu is used as a function outside of a class creation, the "self"
> > argument is not required, so for functions, hence QMenu().
> >
> > For further information read up on class creation of Python (and class
> > instantiation and class instances).
> >
> > Good luck,
> > OldAl.
> > _______________________________________________
>
> You're right about this stuff for python, but wrong about in this usage. In
> this case self is the parent object that owns the QMenu() child. It's not a
> method of the parent object, but an attribute.
>
> i.e.
>
> class MainWindow(QMainWindow):
>  # your explanation
>  def __init__(self, parent=None):
>     # his usage.
>     self.menu = QMenu(self)
>
> Using QMenu() without self, just sets the menu's parent to None at
> instination.
>
> The problem is appears to bein the destruction of the parent. I'm not sure.
> It
> does sounds like the parent is being deystroyed before the child (like the
> discussion on the list a few months ago). I can't remember the subject or
> the
> fix, but I think the fix was to set the child objects parent to None before
> it
> the parent was destroyed.  This is an _I Think_.  Others who participated
> in
> that convo would be better at answering it.
>
> Mike
>
> --
> They are called computers simply because computation is the only
> significant
> job that has so far been given to them.
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