python nested function memory leak?

Question

from a previous experience in java, I am aware that if two GUI class objects contain member variable references of each other, there can be a memory leak until both objects are destroyed.

That situation was very messy to debug and so I want to work with strict guidelines to avoid memory leaks now in python.

I currently have a GUI system where there is a main window (parent_window) and many sub windows (child windows) that pop up with ancillary options for the user, etc.

I thought an easy way to prevent memory leaks is don't keep a class member variable of the parent window in all of the child windows.

class child_window(object):
    def make_child_window(self, parent_window):
        def on_ok(): parent_window.show()
        QtCore.QObject.connect(self.okbutton, QtCore.SIGNAL("clicked()"), on_ok)

as you can see, I have a nested function that calls the parent_window which is contained only in the local scope of the class method ("make_child_window"). Is this cheating? On a lower level does python essentially store my "parent_window" as a class variable? What are your other recommendations for isolating the child_window from holding onto the parent_window in memory?

See [Should I worry about circular references in Python?](http://stackoverflow.com/questions/2428301/should-i-worry-about-circular-references-in-python). — Steven Rumbalski, Aug 07 '12 at 19:44
In particular, I think [this answer](http://stackoverflow.com/a/2428888/600110) has all the info you need. — Sam Mussmann, Aug 07 '12 at 20:12
helpful, though his post was not as technical as I would have liked. If python has a more sophisticated (or better tuned GC for circular references of GUIs) then great. Otherwise Ill need to wait till I finish this first project to get a hang of how capable it is. — ecoe, Aug 08 '12 at 15:36

Luka Rahne · Answer 1 · 2012-08-07T20:12:19.073

you can call parent as it is own/derived method

class child_window(object):
    def make_child_window(self): # no need for parent
        def on_ok(): self.show() #no need for parent
        QtCore.QObject.connect(self.okbutton, QtCore.SIGNAL("clicked()"), on_ok)

This works because self instance is dictionary of methods and contains all methods of all derived classes so you can access them from any scope.

This also answers your first question about GC which is that there is only one reference and self for child is also self for parent and are for derived classes same.

Even further. From child you can call parent_object.show(self) as show(self) would be static method having paramether self to explicitly tell from which class to use method on instance self.

self is just instance

so are you suggesting to show the parent window call the method statically? Would you be able to show an example in code? That example to me appears to be only showing the child window (maybe I am misunderstanding). — ecoe, Aug 08 '12 at 15:40

score 0 · Answer 2 · answered Jan 16 '13 at 18:00

I have found weak references to be an excellent solution.

This means that a "parent" GUI window contains strong references of subsequent "child" GUI windows and, as necessary, each child contains a weak reference of the parent.

Then, when the parent is removed out of memory so is the child as no circular reference exists.

python nested function memory leak?

2 Answers2