ptmax | maximize size of partition if possible | Cloud Storage library

I am coding a GUI program now and I have two separate PyQt5 widget objects that need to communicate with each other. I have something that works now (which I have provided a simplified example of below), but I suspect there is a more robust way of doing this that I am hoping to learn about. I will summarize the functionality below, for those that would like a bit of an intro to the code.

TL;DR: Please help me find a better way to use a button click in object 1 to change a variable in object 2 that sends the coordinates of a mouse click in object 2 to object 1 where those coordinates populate two spin boxes.

This first MainWindow class is where the widget objects are defined. The two objects of interest are MainWindow.plotWidget, an instance of the MplFig class, and MainWindow.linePt1, an instance of the LineEndpoint class. Note here that I am able to pass the self.plotWidget as an argument into the LineEndpoint object, but since MainWindow.plotWidget is defined first, I cannot pass self.linePt1 as an argument there.

The functionality I have achieved with these widgets is a button in LineEndpoint (LineEndpoint.chooseBtn) that, when clicked, changes a variable in MplFig (MplFig.waitingForPt) from None to the value of ptNum which is passed as an argument of LineEndpoint (in the case of linePt1, this value is 1). MplFig has button press events tied to the method MplFig.onClick() which, is MplFig.onClick is not None, passes the coordinates of the mouse click to the two QDoubleSpinBox objects in LineEndpoint.ptXSpin and LineEndpoint.ptYSpin. To achieve this, I pass self as the parent argument when I create the MainWIndow.plotWidget object of MplFig. I set the parent as self.parent which allows me to call the LineEndpoint object as self.parent.linePt1, which from there allows me to access the spin boxes.

This seems like a round-a-bout way of doing things and I'm wondering if anybody could suggest a better way of structuring this functionality? I like the method of passing the MplFig object as an argument to the LineEndpoint class as that makes it clear from the init method in the class definition that the LineEndpoint class communicates with the MplFig class. I know I cannot have both classes depend on each other in the same way, but i would love to learn a way of doing this that still makes it clear in the code that the objects are communicating. I am still open to all suggestions though!